well.. I am glad someone's got the same problem as I am. Now I know I am not the only one.
Igot same as you: (you got even better Motherboard)
Athlon xp 1800+
Geforce2 mx 400 64MB
I always get ~350fps with glxgears and other gl apps are really slow.
I tried everything.. upgrade kernels, nvidia drivers, and XFree86. Nothing works. so fustrating.
Nvidia please fix it.