I'm on an Athlon XP 2100+, 768mb DDR2100, and a Geforce 6000 MX AGP 128mb. Latest nvidia drivers (7676), latest xorg (6.8.2), etc. glxgears gets ~840fps, Enemy Territory and StepMania get <10fps, and mplayer is too slow if it uses -vo gl or gl2. I'm not even going to bother trying HL2.
Oh, and if it helps, I had this same problem with my old GF4 MX420... Didn't bother trying to fix it then, but now that I have a better card, I thought I may as well put it to work.
I was told by someone on IRC to downgrade drivers to 6629, but I couldn't get it installed on 2.6.12-ARCH/gcc 4.2 either with Arch Linux's package or the official installer... Either my kernel or gcc version must be messing it up. I don't know what else to do, so I'd really appreciate it if someone could give me a suggestion or two on how to do anything with OGL.
I uploaded nvidia-bug-report.log to http://chaoticevil.ca/nvidia-bug-report.log.txt
-- it has a lot more detailed information.