The M-Audio Delta 1010LT is described as a 10-In/10-Out PCI Virtual Studio and has provided the backbone for many VTPO installations. It's an easily installable PCI card that will fit into any PCI slot on a PC motherboard, and it is possible to use up to four of these cards in one PC, which would provide up to a maximum of 40 audio channels! It interfaces with most VTPO software without problems and some versions of driver software even allow for separate bass routing and control. The 1010LT also comes with a MIDI interface and even has Surround Sound capability, for those who want more out of their sound card than just VTPO.
Artisan Instruments has recently announced an upgrade to software for its Micro-Midi Control System, to enable it to work with the 1010LT, which is a much welcomed improvement over its previous single stereo channel, when using its sound engine as a sample source. Given that the Micro-Midi Control System is based on the use of Linux, this can only be good news for Linux users and the 1010LT. Artisan Instruments are now supplying a hardware sound engine which also uses the Delta 1010LT.
Fortunately it has recently been announced that fully working 64 bit drivers for it are now available, so memory hungry applications such as some iterations of Hauptwerk can take advantage of main memory over 3Gb, because the 1010LT can now run with the 64 bit operating system required to address larger memory sticks. Drivers can be downloaded here, and there are even drivers available for Windows 7 (32 and 64 bit).
As with many computer parts, prices vary, but the 1010LT is widely available throughout the World and entering the words "Delta 1010LT" into Google should bring plenty of results.
I have recently installed these sound cards as a pair, using the latest 64 bit drivers for Windows 7, and have had no problems.