Apple has today quietly released a new graphics upgrade kit for Mac Pros released before the most recent revision.
The latest Mac Pro revision added the NVIDIA GeForce 8800GT as an upgrade but was not compatible with previous generation Mac Pros due to firmware issues.
The $279 upgrade kit appears to differ from that in the Early 2008 Mac Pro as Apple notes the following on the product page:
Note to owners of Mac Pro (early 2008): Please select the NVIDIA GeForce 8800 GT (early 2008) Graphics Upgrade Kit for Mac Pro.