This is not a recommendation. This is what I have because I used budget parts and what I had leftover from upgrading.
I have around $250 in each PC, not including the OS.
I was limited to a 90w CPU by the budget mobos.
All unnecessary services are turned off in XP, including networking.
Main setup (x-arcade in front of a tv): XP 32, 2.9Ghz Athlon X3, 2GB ram, 500GB Seagate, 8600GT - Plays Tekken 3 with no sound skips. Chokes on NFL Blitz (XP64 would help).
Driving Cab: XP 64, 3.4Ghz Athlon X3, 2GB ram, 500GB HD, 8600GT video card - Plays California Speed, SF Rush at near 100%. Chokes on Ridge Racer.
New Cab: Vista 64 (for directx 11), 3.4Ghz Athlon X3, 4GB ram, 500GB HD, 1.5TB HD, 8600GT video card - Haven't installed MAME yet.
My Alienware m14x laptop with an i7 plays Blitz, Gauntlet Legends, & Ridge Racer at 100% when it's not in for service.