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Radeon HD5770 is a good option. You can get an Apple official one (not "supported" but works fine), or save yourself some money and get a PC one then flash the ROM so it works on a Mac.
 
fyi, I don't think any 5770s have been flashed to run perfectly on Mac, they all don't show the boot screen.
 
fyi, I don't think any 5770s have been flashed to run perfectly on Mac, they all don't show the boot screen.

Which to some, isn't a big deal as long as the card works otherwise. A PC 5770 is a good $100 cheaper than the official Apple card.
 
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