I just upgraded my V5 PCI (actually still using it as a 2nd card) and it runs on par with the GF2 MX AGP. Its lower in some games at 16-bit, but not by much. Also, you can use 2x FSAA without as much performance hit as most cards if you have a fast CPU, as this card wasn't designed to give blazing fast FPS with any CPU. I was getting ~140fps at 1024x16, ~120fps at 1024x32, ~110fps at 1024x16 with 2x FSAA in Q3. Still by far the best FSAA I've seen, and it ran every game I threw at it with very good performance (anything over 60fps is fine IMO).
Finding one is the hard part. These cards are still in high demand for people such as yourself that don't have an AGP slot, and those who do have one usually won't part with it while they own their current system. The AGP version is much easier to come across b/c of people upgrading to GF3's and Radeon 8500's. Your best bet would be to find one on Ebay, as resellers and retailers liquidated their stock shortly after 3dfx went under. I got mine on Ebay when they weren't in such high demand in June for $100 bucks...I think the going price for one now is ~$135. Still an excellent card, and I still use it for excellent 2D as well as awesome FSAA on some of my older games (D2 looks awesome with 4xFSAA even though I don't play it as much as I used to). I would check the FS/FT forum and see if you can't get lucky and find someone willing to sell it for cheap. Good luck.
Chiz