Let's deal with the actual screen first and follow that with what to drive it with. Most people who have had any dealings with televisions or monitors will already know how to measure the actual size of a screen, but just in case you don't - the measurement given by all manufacturers is from corner to opposite corner diagonally. This of course works for all screen ratios. (e.g. 4:3, 16:9 etc.)
Anything that will display the visual output from a computer and has the appropriate interface and connectors can be used as a screen for VTPO, with certain qualifications. If you just want to perform a setup for a turnkey system (stand-alone console) then even a monochrome monitor would probably work, but because nearly everyone now uses colour to enhance or emphasise their display, the sensible minimum will be a 12" or 15" CRT based (glass) colour monitor that's capable of displaying a minimum resolution of 800 x 640 pixels. With the advent of flat screens, glass monitors are two a penny and you or a friend may even have a couple hidden away in the attic, 'just in case'! Once the setup is complete and you're happy that no imminent changes need to be made, then you can just unplug your (cheap) monitor and hide it away. Of course you can buy a gold-plated screen and hide that away - your choice.
If your chosen organ/s are being seen as a visual representation which will form part of the organ 'definition', then you'll probably want to aim for something a little better, but better doesn't have to be expensive. If you're prepared to put up with the size and weight, a CRT monitor will be fine for all but the most fussy software. However, do make sure that your monitor is capable of displaying the minimum screen resolution that your VTPO software demands. In some cases this will exceed 1280 x 1024 pixels and even some of what were the more expensive glass screens just can't cope with that.
Size of screen is really a personal choice. Some folks will be happy with a small screen, some won't. Fortunately, as technology advances, the larger screens are getting cheaper and where you might once have settled for a 15" CRT, you probably won't pay more than the equivalent for a 19" or 20" flat screen. Also, most flat screens if carefully chosen will happily display at 1280 x 1024 pixels and this would seem a sensible maximum resolution to aim for, provided that you're not visually disadvantaged. You should also check with your sample set/organ definition supplier about the screen shape that the organ definition was designed around. If the definition was designed for (say) a resolution of 1024 x 768 pixels on a 4:3 ratio monitor, then it will look 'odd' and unprofessional on a 16:9 ratio wide screen monitor and if you try to make it fit, the display will look stretched and misshapen. The only time you might get away with using a '4:3 organ' on a 16:9 screen is if you increase the final display resolution from the computer and turn the screen thru 90 degrees (portrait mode). As with all of my bright ideas there's often a snag, and this one's no exception! There are not many monitors that can be turned 90 degrees on their stand and you certainly would not want to do that with a CRT!!!
Consider this: You could even use a screen to display your sheet music and use automatic page-turning software. No problems with low light levels and no paper dropping off the console, just when you reached the 'hard bit'! Where your audience is not too far from the console but can't actually see the keys, you could have a camera connected to display your performance, provided that you don't need to see the screen for the status of your VTPO.
When it comes down to final choice, it's the user that matters, not the latest fashion, so choose what fits your intended environment, but do bear in mind that if you are to give performances on your VTPO, your monitor/s will also be seen by your public, so the one with the cracked case and the boot marks up the side is probably not the wisest thing to use!
There is another technology you may wish to use, and that's a Touch Screen (or two). There are distinct advantages to using touch screens to display the entire stop tab, piston and perhaps expression control of your VTPO, and a few disadvantages.
The disadvantages: Any screen parked on top of (or very close to) a Theatre Organ console will make the organ look 'futuristic' and not 'in the old style'. Some organists will find it hard to accept a near-vertical touch method of control over the conventional SAMs (Stop Action Magnets) with tabs. Accuracy of control, especially 'at speed' can be compromised. Unless your software generates 'clunks', there is no reassuring 'thud' as the tabs go up and down. Monitors take up space outside the console. They cost more than a non-touch model (but nowhere near the cost of a full set of SAMs with tabs).
The advantages: Compared to conventional SAMs, they're a cheap option for control. The entire organ can be changed without resort to replacing tab sets or the use of sticky labels! If you're prepared to re-write a GUI representation, the entire layout can be changed without the use of a screwdriver and allowing time for a 're-load' you can use a number of different 'organs' within the space of one performance. You don't even need a conventional console - a keyboard sitting on a stand or table, a PC and amplifier/speaker, and a touch-screen monitor - you have a workable VTPO. Touch screens are useful teaching aids. You can display vital data, musical notation, etc., then go back to controlling the organ.
Touch screens for PCs range from 15" to humungous, but this is one time where size matters! A small screen is fine for 'messing around' but for anything that needs accuracy, especially 'at speed' you need to consider a screen that's at least 17". Having consulted a few users and a few graphic VTPO designers, I've come to the conclusion that a 19" (4:3) touch screen monitor is optimum, as they give sufficient surface area for each virtual stop-tab to be accessed by even the fattest human digit, and they don't cost the rest of your anatomy! As with non-touch screens, size and ratio of monitor is a personal choice and you may prefer something a little bigger. Obviously you can have a screen the size of a house wall and operate it with your fist, and you could even accompany a silent movie displayed on it - that's up to you. What you would do well to consider is the use of two or more touch screens, where two appears to be the most desireable. That will give you space to have one or two 'departments' on each screen, such as Pedal and Accompaniment on (say) the left and Solo and Great on the right. It's also a useful setup if you're into classical organs as the larger ones tend to have drawstops on jambs each side of the manuals.
There are several technologies that are popular for touch screens and which work well for VTPO, some just a little better than others. Infra-Red, which is more suitable for the 'large screen'. Laser, which works well for all screen sizes. Resistive and Capacitive, where Resistive is generally more accurate than capacitive and is the better choice for VTPO work unless you intend to use a stylus. Prices vary between the technologies and from vendor to vendor, but for 20" (and under) screens on VTPOs, Resistive Touch would be a good choice at a sensible price. You will also find that some vendors will supply kits of touch technology that can be attached to existing monitors, both CRT and LCD etc.
A word of warning with touch screens. Not all touch screens work with 64 bit operating systems, even though some suppliers would have you believe that they do! If using 64 bit, check carefully and obtain a written assurance from the supplier that they will make a full refund if their product does not work as stated. The problem is that some monitors will display just fine, but the touch utility doesn't work, usually needing USB 64 bit drivers, which are not supplied.
Below is a list of a few suppliers of touch screens and touch kits.
(Please note that this list is provided as a free service with no endorsement from this website, except for the first one, where I've had first-hand (good) experience).
Graphics capability is a must at some time or other on any PC, even for simply setting up before just using the organ console and nothing else. Any graphics card or facility built in to a motherboard will have a basic output without the need for drivers and this should give you a minimum resolution of 800 x 600 pixels, which will be good enough for setting up in most cases. If this is all that you need, then you have it easy and can ignore much of the rest of this article. However, do remember that current PC screen connectors are either D-Sub or DVI with some of the newer ones including an extra port for HDMI. If your chosen manufacturer has not fitted both types (which is mostly the case) then do make sure that you have either the correct cable to make the connection, or an adaptor to convert DVI to D-Sub, depending on what monitor and cable you've chosen. Note: An adaptor does not convert the digital signal to RGB, it just allows extra connections for RGB that DVI doesn't use. For any graphics card, preferably choose one that is 'fanless' (uses heatsink technology for cooling) as graphics card fans are notoriously noisy and that's the last thing you want whirring away through your music!
For the more advanced graphics features that most of us use, there are currently two major contenders - ATI Radeon and nVidia. I don't intend to get into the war over which is best, but I make one comment that could affect you but hopefully won't. ATI tend to use 'Catalyst' software to control particularly the more esoteric functions on their latest GPUs. This works very well except that they use control codes (the CTRL key and one other key in combination) to select certain functions, and some other software also uses the same codes for its purposes. Neither side is right or wrong, but there is a potential for conflict, and you should be aware of it.
If you've chosen a DVI only monitor/s then you will need a graphics card that outputs DVI, so it will come with the right connector/s. All you have to buy extra is the connecting cable/s. If you have an analogue system, then it won't matter whether your graphics output is on a DVI or D-Sub connector. You can use adaptors if your D-sub connector won't fit.
Do check with your VTPO software/sample set supplier what screen resolution is expected to be used and ensure that your chosen graphics system is capable of displaying it. Certainly some Virtual Organs for use with Hauptwerk require a resolution of 1280 x 1024 pixels and anything less than this may be misshapen, look odd and can be downright difficult to use. There is at least one ATI card that has resolution capabilities below and above 1280 x 1024, but refuses point blank to output the one that you actually need! You have been warned!
Currently, PCI-Express is all the rage for graphics (and some sound systems) but like everything else 'computer', this will probably have changed by the end of this article! AGP was in vogue not so long ago and some older motherboards are set up for this and do not have PCI-Express capability. This won't affect you with a new motherboard, but if you are using an older type, then you may find that suitable AGP graphics cards are getting a bit scarce these days, however when you do find one it should cost very little. What I do advise though is that whatever graphics card you choose, you buy one with as much on-board memory as you can afford, with a minimum of 256Mb. An older system can be 'perked up' no end by changing the graphics card to one with (say) 512Mb on-board memory, relieving the 'bottleneck' which occurs when the main processor can't pass on graphics tasks to a GPU that can't cope. One thing that does seem to have stayed consistent for a while now, is the availability of standard PCI and for some older motherboards, this may be a way to go if you can't find a suitable AGP card.
If you do have a motherboard with a PCI-Express slot then if you're not using on-board graphics do buy a PCI-Express graphics card, and the same advice goes - as much on-board memory as you can afford. However, for a VTPO PC, your PCI-Express card doesn't need to be the fastest thing on earth and you don't need to pay a fortune for a card that was really designed round gaming and needs the ultimate in processing capability, but if you will be using a 64 bit operating system, do ensure that any graphics card you choose comes with 64 bit drivers, or that they are downloadable from the manufacturer's website. If you are going to use Windows Vista and you want to use enhancements such as Aero, then do ensure that your graphics card is 'Vista ready'. Some older cards will work with Vista, but just can't make use of the extra functionality that it now provides.
By now you should have decided whether you are going to use more than one monitor, and if that is the case then you will need at least two graphics output sources to match. It is more the exception than the rule at present to find a motherboard with built-in graphics capability that has more than one D-Sub or DVI output, although some may have one or the other, plus a HDMI port. Don't be tempted to use a HDMI port for a second monitor as Windows only allows you to have one screen resolution at a time, so you can't use the lower resolution D-Sub/DVI with the higher resolution HDMI. Whilst it's feasible to use two or more separate graphics cards in one PC, this will take up extra I/O slots and if you only have one PCI-Express slot, this will make things very awkward. If you're using just two monitors then preferably buy a graphics card with two outputs (dual head) and configure it from the Desktop to split Windows' output across both screens. (See your operating system manual for instructions on how to achieve this).
You should not have any problems with touch screens when using two of them at once. Windows will calculate the mouse pointer position from the data it already has for the screens and the 'touch' software will convert touches to mouse position and will allow you to calibrate this during touch screen setup.
As a final aside, if you are in the habit (like me) of using more than one PC at the same location and don't want the clutter of extra screens (or the expense) you might think about buying a KVM (Keyboard, Video and Mouse) Switch. This will allow you to switch two PCs between a single monitor, keyboard and mouse at the press of a button. These are now available for DVI, so just ensure that you have the correct version for your graphics system. Another little trick you can use if you buy a monitor with both D-Sub and DVI connectors fitted, is to connect one PC to each input and use the monitor's built in menu to switch between them. If you're really lucky, some monitors will automatically switch when they detect a change of signal.