I've also been wondering this, however I don't know how many people have themselves tested XP's against P4's, so it's hard to get a definitive answer. But my research led me to the XP, which I am about to buy, for one main reason: floating-point calculation. It's supposed to be much faster in this type of processing than P4's and music software uses a lot of floating-point calculation, particularly cpu-intensive things like plugins. Again, I've never actually tested any of this, just what I read. I also read that the XP's run cooler than the sometimes very hot Athlon T-birds, which is why I'm going with them, plus the bang/buck, fwiw.
