I recommend WIN2000 - Linux - WIN98 boot system - if you have the HD space to do all 3 - or do what I do and build a network of computers running everything:
Win2000 - the best for highend stuff - very stable
Win98 - the best for games - everything else sucks/very unstable in a work enviroment
Linux - Mandrake - great for working on
Linux Debian - server -
MacOS - 7.5 - MIDI stuff
MacOS - 9.2.2 - Graphics, video, Sound, 3D (Strata, Pixels3D, Lightwave)
MacOS - X - Wont buy anything yet as its still too slow but lots of fun to play with- Free DEV tools is really cool - Does Blender 2.25 run faster??? I know there are some issues with older macs which might be a problem