Whats the big diff in the 256 and 512 BFG video cards

Why does everyone protest getting 2, 512 BFG video cards, and reccommends getting the 2 256's? Well, I did just think of a reason but it's likely wrong, umm, the video card has a certain amount of video processing power and it can push 256 easier than 512 but that can't be true because what company would knowingly make a video card that didn't have enough processing power, but anyways, can someone tell me the beef of the 512 and the 256 BFG 6800's?

There is currently nothing on this planet [ game wise at least ] that requires two 512Mb GPU's in SLI.

Check all of the speeds and frequencies, tell if you dont see a difference, post links. I have heard the smaller the memory, the faster the clock.

So asides from the reason that it is really not necessary there is no downfall to having 2 512 video cards?

Bit of a waste of money aswell if nothing makes use of those BEASTS.