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Just installed Gears on my iMac this morning and it runs fine. Have it running under XP. It looks good and there doesn't seem to be much in the way of slowdown or anything and Ihave the settings up pretty high.
 
The game ran just fine on my iMac's Radeon x1600 256MB (2GHz Core Duo, 2GB RAM, Windows Vista).

It should run really well on the 8800.
 
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