It goes like this:
GeForce 256 32MB DDR RAM - core clock 120Mhz - memory clock 333Mhz (166Mhz DDR)
The oldest model still available. Nice performance, quite affordable.
GeForce 2 MX 32/64MB SDR RAM - Core clock 175Mhz - Memory clock 166Mhz
Nvidias value chipset. Performance is a bit lower than that of the GeForce 256. The same goes for the price. As you can tell from the name, it is based on the newer GeForce 2 chipset. This makes for more up-to-date features, like second generation T&L support. Worth to notice if you are considering the old GeForce 256. Very cheap.
GeForce 2 GTS 32/64MB DDR RAM - core clock 200Mhz - memory clock 333Mhz (166Mhz DDR)
State-of-the-art 3D-accelerator. Kickass performance at a reasonable price. I'd definetely recommend this one.
GeForce 2 Pro 64MB DDR RAM - core clock 200Mhz - memory clock 400Mhz (200Mhz DDR)
Identical to the GTS except for higher memory clock. The faster memory provides a significant performance boost over the GTS. The price is a tad higher. My personal favorite.
GeForce 2 Ultra 64MB DDR RAM - core clock 250Mhz - memory clock 460Mhz (230Mhz DDR)
The absolute fastest card available. Identical to the GTS/Pro, just with higher clocked core and memory. Unbeatable performance at an insane price. Totally overkill for most people.
Hope the above helps a bit. Just ask if there's more you want to know...