I five box on one PC. I do get a little lag in town but other than that i'm fine. So mine would probably be the absolute min to operate without getting angry.

Pentium 3ghz

Single hard drive (only partition i have is my OS backup)

2GB ram (i would like to bump it up to 4GB but that would require a new OS)

Windows XP (vista would probably require more... resources hog!! (Please don't tell Bill Gates on me Ellay!!))

Old Radion 1600 w/512MB

I set main on high graphics settings and slaves on far lowest. I also set keyclone to run slaves at max 10fps since i don't look at those screens except to loot.