Ram limitation won't be the thing that out dates the card, it will be the actual GPU performance it self So in that regard a 6gb will still offer no benefit over the 3gb, both would last the same around
Yes 3gb version is good Gigabyte,asus modells are also very good
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: