Slow video with 3D games

I've got a Matrox Millenium II AGP card. The 7.3 setup seemed to recognize it and the normal video display is fine, but I've noticed that when I try to run various games that (presumably?) use 3D functionality - things like the flight simulator and the Tron racing game - the display is unusably slow (it seems to build a frame every 10s or so!). Is my card just too old to cope with new fancy graphics, or have I perhaps set something up wrong? I've tried playing with SAX2, ticking boxes in the "3D Settings" at random (in ignorance), but with no improvement. If it helps, one 3D game is fine. I think it is called Descent iirc. Perhaps this uses different display technology? Any help appreciated. Cheers, -nick