Here is a list of the GeForce cards:
GeForce 2 Ultra-- the fastest, 64 MB, costs $400
GeForce 2 Pro -- 2nd place, 64 MB, costs $300
Geforce 2 GTS -- 3rd fastest, 32 and 64 MB versions, 32MB costs $200
GeForce 2 MX -- budget card, 16 and 32 MB versions, $100-$150
and the older technology...
GeForce 256 DDR -- slightly faster than MX, costs more than MX
GeForce 256 SDR -- slower and costs more than an MX
The MX is the best bang for the buck. If you go that route, then stay away from the Asus 16 MB version or the Creative Labs DDR version. Go for a 32 MB SDR version.
The Geforce 2 GTS is a great card. Go for the 32 MB version as upping to 64 MB does not help all that much.
Skip the older technology. They lose in the price/performance arena.
Personally, I have an ATI Radeon.