Oddly enough I've only ever had a real issue with screen tearing if I turn VSync on, price wise you can't beat a pair of 6950 particularly if you don't mind voiding your warranty and flashing them to 6970s.
As for the 6990 and 590, as with all top end computer equipment they cost allot more then what they should if it was based on performance alone, however you likely can't go wrong with ether. Just bare in mind both cards though half there heat back into your case so it needs to have good airflow, also note the 590 costs more but it is more powerfully however it is also quieter.
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: ) Skid : )
My Multi-Monitor Focused YouTube Channel:
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http://www.twitch.tv/SkidInc"Imagination is the only weapon in the war against reality."