Anything Really. 300Mhz and up, even a K6-3 would be OK. (a K6 450 would be perfect IMO)
128 megs ram
100bt nic
Thats the basics. Now for storage, if its important, you could go with a raid setup with back-up. If speed is important, go for a scuzzy like thingy drives and a interface card (unless the MB has it).
Dont know much past IDE, but there are several people here that will help ya pick the proper drive.