qbsinfo :
a single 770 does not have enough horsepower to use 4gbs of vram. a SLI setup would provide more horsepower to run a 2d surround set up.
if you can't get a 4gb SLI set up then go for the 2gb.
You said that a single 770 does not possess enough power to use 4GB.So why they offer you with more vram?
Definitely for playing at high resolution.
and if he have enough money to sli 770,then he should avoid sli configurations and have a GTX 780 3GB.