Buying a 2600 would work great. If your going to OC it. Those processor OC very well. With a moderate cooling system, you can jump around 500mhz. But, if your not going to OC the XP's, then i wouldent take it too serious about getting one. But of course, it all depends on what your computer will be used for.
If you seek striaght speed, along with gaming. Then a AMD64 would be ideal. Above all, i suggest getting a mid-range price processor, then OC it. You can spend around 100$ for a coolng system. New heatsink/cpu fan, few 120mm fans.
Depending on the material , you can drop your temps around
30-20 degrees celcius. That leaves alot of room for OC.