AMD Mobile XP with Abit NF7-S

This combination will allow you to overclock your the AMD Mobile XP processor quite a bit because it is not locked.
If you are using an AMD XP procesor (Barton core), then a motherboard that is based on the Nvidia's NForce 2 Ultra chip will be your best option. P4 works best with motherboards base on the Intel chipset (there are quite a few combination).
If you want a high-end system, then the AMD 64bit processor should your top choice since it has pretty good benchmark performance for most 32bit applications as well as having the ability to run 64bit OS. Unless you want a computer system for graphic application and media encoding, an AMD system will give the best bang for your bucks.