Just a guess, but since the 4850 is not a build to order option on the 21.5" iMac, chances are the 4670 is not the same card as that in the 27" iMac. I personally would not be willing to risk the cash on trying one, but if I had access to a 27" iMac with the card I would give the swap a try. This upgrade may not be thermally compatible as well, otherwise I would think Apple would have made it an option. After all, if it just boiled down to fitting the better card in there, Apple would be all over the additional profits they could make off of it. In the case of the GeForce 9400M, it is part of the chipset for the computer. It shares RAM for itself, but when you install another card like a 4670, the 9400M is disabled, freeing up the RAM. The 21.5" iMac with 4670 graphics still has 9400M graphics inside, it is just not used. I would like to see someone pull their 4670 card to verify the 9400M kicks in and takes its place.