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.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w