If you have a Via 686 southbridge, definitely get SP2. It fixes some problems with the AGP bus or something like that (better, stabler, faster graphics)
Yesterday I reformatted my PC, and found that right after boot, if I open the start menu, then the control panel, it would take 15-20 seconds before it opened. The system taskbar would be frozen for that time, but My computer still worked. Upgrading to SP2 fixed this; I think a file that was installed during win2k setup was corrupted when it went to my HDD. SP2 updates the system files, and it doesn't seem to slow the system at all. If you don't mind downloading 15mb, by all means, SP2 is worthwhile.