Coming through on Steve Jobs' recent email promise, Apple released a new graphics upgrade kit today. The $279 NVIDIA GeForce 8800 GT (1st Generation) Graphics Upgrade Kit for Mac Pro appears to finally offer a video upgrade path for previous generation Mac Pro owners.
The card is distinct from the previous Nvidia 8800GT upgrade kit which was available only for Early 2008 Mac Pros:
Note to owners of Mac Pro (early 2008): Please select the NVIDIA GeForce 8800 GT (early 2008) Graphics Upgrade Kit for Mac Pro.
When Apple released the Early 2008 Mac Pro, they offered the NVIDIA 8800GT as an upgrade option, however due to firmware issues, the 8800GT was not compatible with previous generation Mac Pros -- until today.