XP with all the bloated graphics turned off is essentially 2000 with a few refinements. I've used both, and I would say XP is slightly more responsive, however it is also a roughly 2 gig install to 2000's 600 odd megabytes. XP also likes to have a GIGANTIC page file on hand, which has never really made much sense to me.
That said, I would recommend XP at this point, as 2000 is nearing the end of its lifespan. I still consider 2000 to be the high water mark of Microsoft's OSes (XP, as I said, is still 2000 as far as I'm concerned) though.