Well is it really all the resolution that makes the difference?
It really depends on the texture quality, right? In some games you can set texture resolution. If you set texture resolution to 'Ultra High' or what ever, then it will use more vram. If you set it to low, it will use less, and regardless to what you set it to if you have enough vram to support your setting, the FPS at high texture resolution will be the same at low texture resolution.
So if someone buys a gts320, he can play the same games the gts640 can, except the gts640 can utilize high resolution textures if the game supports using them at the same FPS the 320 can. If the gts320 plays at high resolution textures, the game is going to be choppy because it's constantly having the find stuffs on the hard drive.
Am I right?
edit: either way, gtx owns all. 768mb ftw. no worries.