What type of NVIDIA card do you have, and which drivers are you using? (I assume the most recent NVIDIA beta 5338 driver release?)
When you run glxgears in a console, what framerate are you getting?
You mind find it worthwhile to try some of the hacked NVIDIA drivers from http://minion.de/
-- they're unsupported, so use at your own risk. I've tried them, and they installed and worked fine for the most part. The upshot of these drivers is that they're repackaged to work with kernel 2.6, so you can try the NVIDIA 4496 beta instead (which work great best with recent Mandrake distros, IMO)