Voodoo 3 PCI cards are getting very hard to come by at decent prices.
They are desirable because there are much better\faster AGP cards out there for systems that will be playing a variety of games, but if you want a good Glide experience on the same machine you'll need a PCI Voodoo. The most affordable, reliable and best performing cards to get are the Voodoo 3 series. A Voodoo 2 SLI setup is competitive performance-wise and is flexible but its more work, more expense (usually) and can reduce 2D image quality. Much easier to just get a PCI Voodoo Banshee, V3 or if you feel like spending an arm and a leg, a V4 or V5. The Voodoo 3 is also known for its high quality 16bit color due to the internal 24bit rendering it does. So games that don't support 32bit color can look better on a Voodoo 3.
I would combine the two systems into the Athlon. Use the best parts of both, use the Geforce 2 MX (assuming it is AGP) and the Voodoo 3 PCI if possible. The P3 is probably a little faster, but generally anything that benefits from that will probably run even better with no problems on a much newer system. The Athlon would be a nice 98SE system for playing games up to 2000 or so. Lots of Glide support and some early 32bit color games that the Geforce would be better for. It would be easiest to dual-boot two installations so that one OS is read to go for the Voodoo and one is ready for the Geforce with the other card disabled in each OS. That would make for an awesome system!
Now for some blitting from the back buffer.