Nvidia has released a new suite of graphics drivers for the GTX-285 graphics card. The driver is listed for OS X 10.6.4 only, and is specifically for the GTX-285 card that is available as a third-party purchase for Mac Pro systems.
The update specifically addresses OpenGL performance issues and improved compatibility with the latest OS X software updates. In addition, the update has support for applications that implement Nvidia's CUDA technology, but you will need to download and install an additional driver for this functionality.
Keep in mind that this driver will replace the one provided by Apple, and while it should bring new and updated functionality to the GPU, there is a chance that you may experience some errors. If so, try booting into Safe Mode and using a maintenance application to clear the system's boot caches, along with running other general maintenance routines.
Be sure to have a full backup of your system before updating, so in the even of unforeseen problems or incompatibilities you will be able to restore your current installation.