I agree that the 3GB version is enough. The 6GB version comes with a $150 additional cost, a decent factory OC that will boost performance a bit, but not enough to warrant the cost.
What I would do, if I were you, is get the 3GB version and use the money you save to get an i5-3570k:
http://www.scan.co.uk/products/29-4intel-core-i5-3570k-s-1155-ivy-bridge-quad-core-34ghz-5-gt-s-dmi-650mhz-gpu-6mb-smart-cache-34x-
and a an Intel-based AsRock extreme motherboard:
ASRock motherboard
You'll definitely see better performance with Intel architecture at this time.