XP is Windows 2000 with a rather garish interface. 2000 is more stable and secure and there are no obvious benefits (unless you are a die-hard gamer) from spending the extra moolah. Better off buying more RAM etc.
It's difficult to say for certain, I recently changed to XP Pro. and had no problems at all, everything worked as sweet as a nut from the start.
But in saying that, I made the change from 98se, so there was quite a difference, and I also never had a problem of any sort with that either. Just that I liked the fresh modern look of XP. I still do and cannot fault it.