The computer, CPU, SoundCard, and MIDI interface you use will depend on what samples you want to run, and your budget. GrandOrgue in Puppy Linux will load the Jeux D'orgues 2 (Stiehr Mockers) in just 512Meg of RAM, and is very playable on a Celeron 900Mhz. It does take quite a while to load because the disk speed and RAM speed on my Celeron 900Mhz is quite old (that computer is about 7 years old). So your older computers will usually run Ok for the smaller sample sets.
As you're wanting to play a VPO with a larger specification than just the Stiehr Mockers, and will want a low low latency playing experience, I'd suggest a dual-core CPU and a Motherboard that has at least 800Mhz FSB memory and 800Mhz rated RAM. How much RAM you need will also depend on what samples you're wanting to run. I have a dual-core AMD 2.8Ghz with 4 Gigs of RAM, but that is because I run an ultra large VPO (over 90 ranks), and run multiple instances of GrandOrgue. 1 Gig or Ram will be fine for a moderate sample set, but I'd be happier recommending 2 Gigs of ram if you're wanting to grow your sample set base. I run Puppy from an 8 Gig USB drive, but it is probably best for a dedicated organ computer to install Puppy on the harddrive. (We run Windows XP 32-bit, Windows 7 64-bit, Ubuntu 9.10 64-bit, and Puppy 4.3.1 32-bit all on the AMD dualcore - I'm a geek!).
For Puppy Linux, avoid the Creative X-Fi cards for now... the driver is not complete in the currently used kernel. Audigy should work fine (almost all soundcards should work well, they just need to support 44100hz 16bit audio rates). I use the onboard Realtek AC97' chip that came with my motherboard (and I also test with a Soundblaster Live! Value card and a CMI8387 card). All the MIDI USB cables that I have tried have worked 'out of the box' in Puppy.
So I guess Generic computers will work very well for GrandOrgue in Puppy Linux. (But then the only generic computers in my house are the laptops!).
Hope that makes sense
GrahamG