ATI Radeon X1900XT 512 MB for mac pro first gen
Alternative may be to get a new machine
For me, the best place for any type of upgrade on your Mac Pro 1,1 (mine is a 2.66) is at Other World Computing.
The only video card available from OWC is the ATI Radeon 5770 HD.
Note: The same video card is also available at the online Apple Store.
I'm hoping to get it soon.