Well You could pick up a 2800+ socket 754 for about 150 bucks. And from what I've read, I could be mistaken.. The Amd 64 vs xp, the 64 has 25% better performance clock for clock than xp. Ha.. the amd 64 socket 754 2800+ is 3 dollars cheaper than the amd xp 2800+.. Trippy..
After looking at newegg.com it looks like you could get a socket 754 2800+ with an Nforce 3 motherboard for about 50-60 bucks more.. Here's link to both.
Cpu - 146 dollars
http://www.newegg.com/app/ViewProduc...103-452&depa=0
Motherboard - 85 dollars
http://www.newegg.com/app/ViewProduc...138-237&depa=0