The problem with wanting faster video performance out of an iMac-style case is that the cooling requirements are pretty strict. That said, Apple gives the option of a GeForce 7600 GT in the 24" iMac when you order online, which makes it a respectable gaming rig if you're not sitting on the bleeding edge. Not that you could fit a GeForce 8800 or an X1950 inside anything that thin.
For that matter, you're probably not looking in the right field if you want an all-in-one but also want extreme performance. I love my iMac because it takes up little space, it's quiet, and it looks like it belongs in the future.