If I can help:
I am running a Pentium 4, 3.0ghz and I can run most of what I want to run, except KI2 has a little stutter that I need to tweak.
I got that PC for free, you can pretty much find one for free or under 50 bucks if you want to go that route.
What are you looking to play? The toughest games to play are the blitz/gauntlet dark legacy games. However from what I understand, if you run a dreamcast emulator, those games play just fine. Also the 3D fighting games, like tekken, don't play that great on my machine, but from what I hear, the model 2 emulator and zinc make a huge difference, but I haven't gotten around to playing with those yet.