I just formatted mine and installed Windows 2000. XP Pro gave it some problems playing certain video files. Win98 cured that, but even with a K6-2+550, it still can't quite play DVD files. It is so close, maybe if I replace my 128 meg SDRAM with a 256 meg stick. W2K seems a good balance for this machine. It can play most Divx files smoothly. And I like it much better than Win98.
The Tweakbios utility does not work with 2K unfortunately. That is too bad, because some of the settings it offered made a real performance difference.