I would say without hesitation that WindowsXP is much more stable than 9x.
As for Win2k, when I first installed it, I decided I wanted to go back to win98 for games, but when I got win98 installed again, I found it to be alot slower than win2k was, so I switched back, and used it for gaming for the next year or so without any problems.
I'm hoping that Microsoft's new Virtual PC 2004 is going to solve my 'oldschool' gaming problems. I'm hoping it's going to be able to run a virtual machine with 98 until XP Pro, but I haven't had a chance to try it out yet. If it has support for all hardware that 98 did, and can run at a reasonable speed with the hardware available today, it should prove fairly useful.