Only way is to switch out the whole logic board with a better GPU. The AL iMacs are all the same, so no upgrades there yet. Powermacs and Mac Pro is the only ones you are going to be able to switch out
The GPU is removable (by a service tech) in the Aluminum iMacs and the white 24-inch iMac, but it's not a standard PCIe card, it's custom-built to fit into a slot on the iMac. There's no way of getting anything other than an identical replacement.
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aluminum imac***
NO
Only way is to switch out the whole logic board with a better GPU. The AL iMacs are all the same, so no upgrades there yet. Powermacs and Mac Pro is the only ones you are going to be able to switch out