changing hardware on a mac is almost impossible. they really are not like pc's where you can just change a GPU and it'll turn right on. Weaselman is right, it will be "locked" in the configuration it came as with a few exceptions. you MIGHT be able to swap the GPU with another one that was an option at the time it was new, but thats about it. I bought a G5 mac pro and read about upgrading the graphics, the options were limited to what came stock with a FEW exceptions, and those exceptions involved flashing a custom ROM on to the GPU and HOPING it works..
if you go to http://www.tonymacx86.com/forum.php
which is a site for building a "Hackintosh" you'll quickly see the hardware limitions and how much work it takes to make something NON-MAC work in a MAC