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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.
 
I'm actually in the boat for the same thing. Is there anything cheaper than the Radeon HD5770? I have a 512 ati x1900XT.
 
Sure, if you want to flash a PC card. The 4870, 4890, 5770 and 5870 are all options.
 
fyi, I don't think any 5770s have been flashed to run perfectly on Mac, they all don't show the boot screen.
 
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