Need to know more information. AMD/INTEL? What socket? possibly what motherboard do you have.
What kind of bandwidth are you pushing?
I run my two home OWS servers(the classic servers) off a AMD x2 3800+ (OC'ed to 2.7GHz). Seems to do fine. But the problem I have is lack of bandwidth and my ISP likes to block certain areas(why it can't be the testing server anymore, wouldn't allow some people to join).
If you can pick up a cheap Dual Core should be enough, but again most the issue most likely is going to be your bandwidth.
Need to get Kujo, or maybe VC to look at this. Kujo runs the Bradley Uni. servers, and seems well educated in server type support.