The 9800GT is better because it has more shaders small vector processors within the GPU that figure out how 3-dimensional placing affects the lighting and color of what's on screen. You cannot change the GPU on the current iMacs. They are soldered onto the motherboard. Wait a bit longer, Apple should come out with iMacs with 8600M GTs' in them. Those cards are excellent for gaming. Not in the range of the 8800, but still more than enough.