Win2k is far more stable than 9x/ME, and so (in my experience) is XP, which would be a better choice if you play games. Both have the advantage that an application can crash without the operating system commiting suicide in sympathy. However, both require care - you may find that hardware and/or software no longer works following the upgrade. For example, my Agfa ePhoto camera didn't make the transition to XP despite it being about 6 months old. Caveat Emptor.
However, if you decide to upgrade (and I'd recommend it) I'd echo SLF's comment that a format+install is the way to go. I've never been very happy with upgrades, as you almost always end up with a bunch of cruft from the previous installation - it never gets totally removed.
[ 12 February 2002: Message edited by: Evo7 ]</p>