I have a brand new AMD Athlon 64 3000+ and it's the best CPU I have ever had. The of a Athlon 64 is cheaper and they run faster than Intels CPU in most cases. Athlon 64 is fully compatible with PCI-Express if you get a mainboard with PCI-Express sockets on it. The choise of mainbord IMO would be Asus or MSI, both HQ and fast. I would buy a mainboard with the new Nvidia nForce4 chipset, it has PCI-Express.

There are good tests of hardware here (CPU, mainboard, ect.):
http://www.tomshardware.com/