I'm citing on one reference, 3DMark2000. For Athlons at 1000 Mhz (or slower). The best scores for a Voodoo Banshee top out at about 1500 (just for reference). For Geforce256 SDR, Geforce 2 MX, and Geforce256 DDR, the scores are in the mid 5000's, the high 5000's to mid 6000's, and mid to high 6000's (one score reached 7022), respectively.
Note that many of those scores are probably obtained with BIOS tweaks and video card overclocking but it does give you a direct comparison for your Thunderbird class machine.
Based solely on the 3DMark 2000 results, the Geforce II MX is probably the best value since it performs in the middle of the 3 but costs the least. The other 2 are still pretty good considering their probable prices. If you have a 300 watt power supply I would consider the Geforce256 DDR for the best performance of the 3.
The best prices, according to www.streetprices.com, for retail versions are; an AOPEN Geforce 256 SDR - $129.95, an Asus Geforce 2 MX $102.95, a Leadtek Geforce 256 DDR - $152.95. OEM versions will certainly be cheaper but probably are not returnable should you encounter compatibility problems.
My card is a Visiontek Geforce256 SDR which I like very much but wish I had splurged and bought the DDR version. Still, I believe I can hold out until after the NV20s (Geforce 3s?) come out and drive the Geforce 2 Ultra cards down in price before I upgrade again.
Incidentally, I run a Duron@900 Mhz and score 4650 without overclocking my video card and 4850 with modest overclocking (which is where I keep it).