I had a 4.4 server running with an AMD 2.4 GH and a Gig of ram, with a cable connection that has 385 Upload.. I was able to run 15 zones and hold 15 people befor things got realy laggy. You can not run a server on a computer that is used for other things, for example, When I logged into my server from the same computer having 5 people on with me was hard and if there was fighting it went down to 3 people.

Easiest way to get a decient server up is with a T-3 and 3-4 computers. 1 running world and a few zones and the other computers running more zones.