Today Apple has released a NVIDIA 8800GT graphics kit upgrade for all mac pros. There was a version of the upgrade kit for only Early 2008 mac pros due to firmware issues. The kit is priced at $279.
http://store.apple.com/1-800-MY-APPLE/WebObjects/AppleStore.woa/wa/RSLID?s=topSellers&fnode=home/shop_mac/mac_accessories/displays&nplm=MB560Z/A&mco=OTkzNDY
http://store.apple.com/1-800-MY-APPLE/WebObjects/AppleStore.woa/wa/RSLID?s=topSellers&fnode=home/shop_mac/mac_accessories/displays&nplm=MB560Z/A&mco=OTkzNDY