There's ~17 steps just to get to the card - not a beginners DIY project...
Officially, Apple provides some models with upgraded video options at the time of purchase, but the video is not intended to be upgraded by a third-party. However, based on disassembly photos and a variety of teardowns, it appears that some models have the video mounted on its own card and it is theoretically upgradable in these systems. Possibility MIGHT BE: 512 MB Radeon HD 5670...
If video expansion is a top priority, you would likely be better served by selling your current unit and switching to a Mac Pro.
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