Mate, I know you've got a thing about this but its really not true that you need all that vram.
Regardless of people's opinion on 'how vram usage works' all anyone have to do is check out the reviews of the 2gb card to see that they are more than powerful enough.
Comparing a 560 to a 680 has issues. Whilst having that less vram could well cause buffering issues on the 560 you also have to compare the capabilities of the processors which aren't exactly equal. Apples to apples etc.
Here's a
review of the 4gb 680 which was specifically set up to test the advantage of having 4gb. I quote from the conclusion:
"The 4GB -- Realistically there was not one game that we tested that could benefit from the two extra GB's of graphics memory. Even at 2560x1600 (which is a massive 4 Mpixels resolution) there was just no measurable difference."
Multiple monitors, probably, but not a single screen. Processor limited first.