I say 500MHz, provided you have 256+ MB RAM.
You might get not too bad performance with 400MHz CPU and 512+ MB RAM, though. Never tried that.
It sorta depends what ya wanna do. If winXP is only to used for email, web surfing and a bit of office software, then 500MHz with enough RAM should be fine. But if ya wanna burn CD's and multi-task, or do any graphics work, then you'll need a faster CPU - say 1GHz minumum.
This is all offa the top of my head from experience, though. Not really reliable.
Cheers
Mick