Well it took a while, but XP installed on my Celeron 466 MHZ computer.
Of course, a problem-free install was not to be. After I installed Service Pack 1, Windows Update stopped working. I flailed around a while trying various fixes suggested by Microsoft to no avail, so then I just reinstalled XP from scratch on top of XP, which seems to have done the trick -- I left update running when I went to work this morning ... I did download a registry cleaner which found all sorts of gobbledygook in the registry so maybe that helped.
I did learn
Another XP anecdote -- I was helping someone at work today, who has one of those cute IBM thinkpads with the 12" LCD. I noticed she was running XP, so I looked at Control Panel/System and it turns out she's running a 500Mhz PIII with 180 Mbytes of memory. It didn't seem that slow either, and it got even faster when I showed her how to turn off all the visual effects. I wouldn't be eager to run XP on anything slower than my 466 Celeron, but I bet you could do OK for everyday email and webbrowsing stuff in XP on a PII 233MHZ with 128Mbytes of memory. It would just take a long time to boot and get the browser and Outlook open. So Microsoft may have done a better job with XP than anyone's giving them credit for.