I used XP on both a P3 1000 and a P4 1.6 and absolutely HATED it. My system slowed down enormously (except for the faster boot time, I'll give it that)
Explorer or IE took more than 15 second to load. What the PC was doing in the meantime I don't know. It also locked up on occasion and I simply coudn't get used to the fact that I had now lost the ability to preview a movie in explorer.

Also, IE 6.0 that comes with XP is much worse than the one that came with win 2000. Only thing I liked about it was the feature to save a picture by moving my mouse over it. But that's it. Otherwise, nothing but troubles. Some sites simply refused to load. (anandtech.com) was one of them. There was always a windows popping up asking me if I wanted to save the page. If I said NO the page simply wouldn't load. Multiple clicking on the link sometimes solved the problem, but most of the times it wouldn't.

So I ended up reinstalling win2000 again. A (more or less - certainly by microsoft' standards) stable OS. Everything is peachy now. I haven't yet found a single thing that I find lacking in 2000. OTOH, when I was working with XP I also couldn't find one single feature that made it worth the upgrade from win 2000. Nah, 2000 is here to stay for me. At least until MS offers an alternative that's worth considering.