I have to go with XP on this one. Sure 98SE is the best for gaming, so if you don't mind dual booting then I would go with that. But, for all around performance it has to be XP.
It is built on the Windows 2000 kernel and loads almost 50% faster! I think it is more stable and faster than Win2000 ever was. Plus, Office has tons of new features!!!
I've played plenty of games on my XP Pro and not had any problems with FPS or lag or anything like that.
Just my 2 cents though...