Win 98SE ... hands down. Win 98 SE has better memory management than Win 95, and has inherent USB support. Win ME is a mess (ask any technician and you'll hear horror stories). The last time I saw an OS with non-Plug-N-Play support this bad was with Win 3x. W2K is definitely not the way to go, the driver database is sparse, and the OS is not optimized for gaming. Install Win98SE, get the latest drivers for your GTS card, install DX8 ... and frag away!