Concerning nVidia 3D settings:

Make sure V-Sync and Triple Buffering are set to "Force Off".

If you wanna take it further, download the utility RivaTuner. In addition to numerous other tweaks, it allows you to set the quality even lower than the nvidia control panel.

Like MSB, I too am running an Athlon 1.8 with a Geforce4 MX. After tweaking various settings to no avail, it turned out that the most effective was to set the Unit Size no greater than Large. Of course you can go lower and then increase other quality settings, but to me anything less than Large just ain't no fun.