I dont know what the OS or driver requirements are, etc.
But just about ANY Nvidia card with 1Gb or more of RAM and 96 or more CUDA cores can be enabled via a small software "hack."
Basically, you just need to edit a line in a text file to enable GPU acceleration with any Nvidia card that meets the above requirement. I cannot say which have successfully worked on older Mac's.
For a desktop PC, that includes a $20 GT 430 card. This site has a lot of detail:
http://www.studio1productions.com/Articles/PremiereCS5.htm
The desktop cards starting with the GTX 550TI and GTX 560 SE at $100 are pretty functional, up to about $160 for the full GTX 460 or 560 will get you to a good cost/benefit point.
Good luck!
Michael
