yes or you get a thing called bottlenecking however most macs are not upgradable (except recent ones) and that will not occur.you should look at the system requirements on the back of the graphics card box or online and see what it requires its always a good idea to have more than that im always well ahead of te system requiremnts when i buy one.
bottlenecking is to any computer at all, its when u say have 8gigs of RAM a 64bit OS a nVidia 6600LE and say a quad core processor, now that would be a GFX bottleneck