The question, though, is if this "512 meg" ceiling is overadvertised by Nvidia and its promoters in order to redirect consumers toward the GTX line.
This "Ceiling" is not an advertisement. And its very real. AA + Resolution eats framebuffer data. Lots of it. Specially beyond 4xAA. And games like Crysis with heavy usage of uncompressed normal map textures will eat up memory like there is no tommorrow. The 512 problem has been there "Long" before the HD4000 series ever existed. And could be seen on 9800GTX 8800GT cards in comparison to 8800GTX cards in specific cases.
As time has moved on these cases have increased. Memory amount can be a hard bottleneck. Specially when used. Once you exceed that framebuffer count. Your system memory starts acting as framebuffer. How ATI/Nvidia may allocate that system memory to framebuffer could very well be different. But the point is you cant get past the Zstorage of AA data. And its expensive.
Just some framebuffer numbers to chew on.
4xAA/16xCSAA @ 2560x1600 250 Megs
4xAA/16xCSAA @ 1920x1200 140 Megs
8xAA/16xQCSAA @ 2560x1600 370 Megs of Memory
8xAA/16xQ CSAA @ 1920x1200 280 megs of memory.
Thats just memory dedicated to Zstorage alone.. That does not include HDR which also increases storage data. It also doesnt include textures. Uncompressed ((or compressed Normal maps)) either. Its a pretty inescapable fact that once you run out of onboard memory. You start touching system memory. I'd rather not do that it any event. Using system memory as a means for storing framebuffer is one of the largest causes of micro stuttering that exists.
I just got finished testing Crysis on 9800GTX + SLI cards. And they cant even produce 15 FPS @ 1680x1050 with 16xCSAA and Very High settings due to running out of memory.