Of course a 3500+ is going to out proform the XP. The next AMD64 after that is a 3700+ witch is mad good. Dont forget XP FSB is what 333mhz 400mhz tops. Since AMD64 chips has its FSB built in the processor it self, its alot faster. The 3500+ has HTT witch helps alot.
Also dont forge socket 939 is the best out there. But for my personal suggestion, dont get a 3500 Newcastle or Win. Get a socket 939 3400 Clawhammer. These will cost around the same price as the 3500+. It has 1mb cache, unlike the 3500 640kb.
Finished With TF.