Memory and Operating System
I would not consider using any PC with less than 1Gb main memory for all but the simplest VTPO. You might get away with less if you only want to run a word processor, surf the Internet and send/receive emails, but you will probably find the system to be sluggish due to the constant need to run swapfiles on the hard drive.
The problem with adding memory to an older PC is that you will often be limited by the capacity and number of slots on the motherboard and a general non-availability of suitable modules. Most memory for PIII and some P4 systems will not be available as new items and will probably come with very limited or no warranty. The one good thing about these older systems is that memory generally comes very cheap!
The ideal amount of memory for such as GigaStudio 3 would seem to be 2Gb but the new GigaStudio 4 may benefit from more than this, especially if used with the GigaPulse VST convolver. Hauptwerk will run on 2Gb memory but greatly benefits from 4Gb and for best performance 6Gb is fine with 8Gb leaving some reserve. However, it's no good buying large amounts of slow memory and you should aim for the fastest memory with low latency that is compatible with your motherboard/chipset.
Always check to see if your motherboard manufacturer makes any recommendations regarding brand and type number. Asus for instance usually has a chart associated with each of their motherboards, which is quite comprehensive. You should stick to their recommendations unless you already know of a brand and type that works that's not on their list. Motherboard manufacturers do actually test memory for timing issues and it's a myth that they respond to 'bribery' from memory manufacturers to advertise their wares. If you buy 'out-of-spec' memory, test it thoroughly and be sure that it's running under the same conditions expected for your VTPO. I've had memory from reputable manufacturers that appears to work fine until you look at its reported conditions and suddenly find that it won't run as Dual Channel (a kind of RAID for memory). There's nothing wrong with the memory modules, but they're just not compatible with some motherboards. As with hi-fi there's a minefield with memory types and uses. DDR3 is now in common use, especially with the latest processors. Check thoroughly and ensure that your chosen memory will work both with your processor and your motherboard.
It's common practice these days to use memory sticks in pairs. This is partly left over from the days when memory would only run as a pair and partly that memory which runs under dual channel architecture also needs to be installed in pairs. Whenever possible, try to run memory in Dual Channel mode as this greatly increases speed performance.
When buying memory, you also need to consider future requirements. It's generally accepted that it's best to have all your memory made by a single manufacturer and that the memory type and size should preferably always be the same. If you buy half your requirement and try to add to it later, you may well run into problems of non-availability, which could mean that you'd have to replace your entire installation and may not be able to sell-it-on without incurring quite a loss.
Another common mistake is to buy memory sticks that are too small in capacity (perhaps because they're 'on offer') and completely fill all of the slots on the motherboard. Then when you want to add memory your slots are full and you have to replace the entire memory block with sticks of a larger capacity. Remember that at present there is normally a maximum of 4 memory slots available on a PC motherboard, so if you need (say) 8Gb of memory, you would need 4 x 2Gb memory sticks. Also, memory prices go up and down with the wind! Your best bet is to buy the maximum amount of memory that your motherboard will take, whilst the price seems at its best to you.
I've included the Operating System here because your choice may depend on the amount of memory that you decide to use. To address more than 3Gb of main memory, you will need a 64 bit operating system such as Microsoft's XP64, Vista64 or Windows 7 64 bit. You won't do any harm by using a 32 bit OS with more than 3Gb memory, but the OS won't use the 'extra' memory and it's wasteful. Some distros of Linux may also take advantage of more than 2Gb memory. Having said that, Linux is a generally 'unbloated' operating system and there has been great success running MidiTzer under Linux control direct from CDs or Thumb Drives.