I am not convinced about the value of extra vram. I suspect the benefits are mostly marketing.
If the designers of these cards needed 4gb, they would have included it in the first place.
As I understand it, if there is insufficient ram, the card has to do a bit more work, but all of that is factored into the benchmarks you have already seen.
I could be wrong.
I have two 2560 x 1600 monitors and a GTX680 with the default 2gb. I see no problems, and frame rates are great.
Sounds good to me, thanks for the advice everyone. I have my GTX 670 FTW on order now from newegg.