In fact 900mhz would still gain appreciable benefit (though not optimal) from GF3TI200 or Rad8500/9000 cards which are now a huge bargain for under $100. Enabling things like AA and Aniso significantly help to use up otherwise untapped GPU speed. Here's a couple of 3Dmark examples using a Duron900mhz (gfx card: total marks, games1-4 high detail FPS):
1024x768x32:
GF2MX400: 2900, 17, 25, 26, na (no DX8 hw support)
GF2TI: 3800, 20, 32, 32, na (no DX8 hw support)
GF4MX440: 3800, 21, 34, 30, na (no DX8 hw support)
GF3TI200: 5500, 25, 49, 40, 33
1024x768x32xAA:
GF2MX400: 1200, 8, 10, 13, na (no DX8 hw support)
GF2TI: 1700, 11, 11, 18, na (no DX8 hw support)
GF4MX440: 2700, 17, 25, 23, na (no DX8 hw support)
GF3TI200: 3700, 18, 35, 28, 20