You can run a GTX-285 on a 1,1 Mac Pro but it will not utilize all of the GPUs performance due to bus limitations. There is a patch Kext file which allows the GTX to run on the older machine.
The GT-120 is a low performance card I think that the 8800 is higher performance. You should take a look at the configuration guide on BMD's site for the Mac-Pro.
-Rob-
Robert Houllahan
Director / Colorist
Cinelab Inc.
http://www.cinelab.com
MAHC-PRO 6-Core 3X GTX285 20Tb SAS Wave Panel Panny 11UK SDI Plasma.