D2X-XL has been built for native 32 bit Windows only (plus Linux and Mac OS X). No guarantee for Win9X/ME - but you can use the original Descent on these (and maybe get a 3dfx voodoo accelerator card for them on ebay to get good gfx).
Everybody who owns a V4 and V5 would be crazy to sell them. These cards have more than just material value. 3dfx understood more of what hardware gamers really need than NVidia or ATI. Unfortunately, the latter had the better marketing and managed to sell features to ppl they didn't really need in the first place, that weren't really available when promised, and that disappeared soon after they had fulfilled their purpose of wiping the competition off the scene. I hope NVidia gets stomped by ATI sometime in the future - they deserve it.