That's true but the trick is that the GTX 680's GPU is not capable of handling such amount of VRAM, basically no GPU around is. So after 3 years there will be GPUs capable of handling this amount of VRAM. But right now if you get a 4Gb version whether it will increase your FPS by 5-7 FPS or hold back your performance, that's because when the additional VRAM doesn't add a value it holds back your performance.
Note that when the additional memory doesn’t make a difference, it reduces performance slightly, a phenomenon already observed with the 1 GB and 2 GB versions of the Radeon HD 6950.
Look at HD 5570 512 and 1 GB