I posted this earlier today somewhere else, both BFG and XFX are carrying 512MB Ultras now.
I don't see why you want to spend to ridiculous extra money for something that is virtually useless. VRAM is simply used for storing textures and the like and considering only one game requires that kind of VRAM, it's a silly investment.
Also, the actual GPU architechture (eg pipeline count, core and memory speed) have to be fast enough actually feed that size memory and make good use out of it. The 6800 Ultra may seem good now (which it is) but when games come out that actually start needing 512MB VRAM (won't be another year) the Ultra most likely won't be able to keep up with the graphics processes.
Take for example a house, you can buy a million square foot house, but without a half decent furnace it's not gonna all stay nice and warm inside.
Especially when you factor in the GV-70 and r520 which are due in about a month that boast supposedly double the performance of even this card.