I've got the same problem with my Geforce 4 MX. Of cource it ain't a that terrific graphics card but it should perform _much_ better. I get a FPS of ~1000 in glxgears @ 16 bit. When i try to run @ 24 bit glxgears gives ~800 fps but GLX runs very crappy, it produces a flickering image
I'm running Gentoo 2004.2 @ my pc, with the 2.6.9-rc2 kernel (i have also tried the 188.8.131.52 kernel but same result) and the newest nvidia drivers 6111
I have read a lot of forums and howto's about nvidia driver problems, but still i haven't managed to solve the problem.
btw, I've got a A7N8X Deluxe mobo with a Athlon XP 2800+ cpu and 512 MB RAM
edit: Sorry for my crappy english, can't help it, i'm dutch :P