After the new Mac Pros were released, you could get an upgraded video card; but for some reason, Apple failed to release this update for the first gen. Mac Pros.  Apparently, Apple has finally made the upgrade kit that will now let you get the 8800 GT.

Specs:

For advanced graphics performance, choose the latest-generation NVIDIA GeForce 8800 GT with 512MB of GDDR3 video memory. With a unified shader core and massive memory bandwidth, the NVIDIA GeForce 8800 GT brings advanced performance to graphics-intensive applications like motion graphics, 3D modeling, rendering, and animation. It features a PCI Express interface for a high-bandwidth connection to the Mac Pro and two dual-link DVI ports for connecting up to two 30-inch Apple Cinema HD Displays.
Requires Mac Pro (bus speed 1.33GHz) with PCI-Express
Includes a DVI to VGA connector for easy connection to VGA-based displays
Note to owners of Mac Pro (early 2008): Please select the NVIDIA GeForce 8800 GT (early 2008) Graphics Upgrade Kit for Mac Pro.
Grab one HERE