Go for an AMD Athlon 64 3500+ with the Winchester core (90nm), and a MSI K8N Neo2 Platinum.
http://www.xbitlabs.com/articles/cpu/display/athlon64-90nm.html
http://www.anandtech.com/mb/showdoc.aspx?i=2128&p=10
Edit: Microsoft haven't released Windows XP x64 Edition yet, but you can...
Don't mix up PCI-X and PCI-Express. It's two different buses. PCI-X (or PCI-extended) was made by IBM, Compaq and HP back in 1998, I think. Anyway, back to your topic. :rolleyes:
Try to save just a little more for a Geforce 6600GT.