How bad are we talking in terms of limitation? The only real games I play across the 3 screens are racing games like rFactor 2 / Project C.A.R.S that aren't so graphically intensive.
There will be very little limitation. The only reason to ever get a 4GB 670 is if you have them in sli. If you are running off of a single card, you will see very little difference in terms of performance. Even if you sli, the cards will still run great, but with a minor bottleneck.