Ok, first - what do you use your computer for? If you play games, then just say no to 2000.
Next - what are your computer's specs? If they arn't to a certain level don't bother running XP, as it will only make your life hell.
3rd - XP is much more stable than 98se, and has more features. But it's also really bloated, much more than even 2000. It can be a good OS for some, and a crappy OS for others. Do you play with networks? Then I would suggest upgrading to XP.
But the main thing is - if it works, why change it? Do you find something missing from your computing experience, like a really horrible implementation of PCAnywhere built into the OS?

If it works fine, there really is no need to upgrade. My experience with 98se has been crash-filled and, well, crash-filled. But that's because I was using 2 eMachines', so ignore that part.
I say evaluate your needs and your computer, then go from there.
-Fox