The pentium 4 or the pentium D will be able to handle about half the load your Xeon server did. If you didnt uise your Xeon to the max or anything you will probabnly be just fine with the pentium D,4 server.
Can you give me an idea what you want to run on it and I will be better able to tell you what is possible.
1 ET server (q3 based) 16 man - private
1 css (source based) 14 man - private
1 cod2 14 man - private
1 RTCW (q3 based) 16 man - private
Future:quakewars - public (not important if need be)
Future: quakewars - private
With those servers, you should be just fine with either the P4 or the PD. The only thing you may have to do is add more ram to the P4 setup. 2 Gigs of ram would be a good thing to have. As for the cpu, you should be just fine.
The only time you may lag the server is when you try to launch all the game servers at the same time.