It would be nice to know exactly what model Mac you have. There are some models that you can not upgrade.
The other clue is the minimum requirement. You will get slow results on those games if you don't have the correct video requirement.
I bought a game that only stated the requirements as G3 or better. But when I went to play the game, I had a black screen. It turns out that there was more requirements listed on their web site that specified the exact video card - which I did not have at the time.
Now I do