People keep recommending 2gb of VRAM for 1920 x 1200 resolutions, but the evidence doesn't hold up. Check any review showing a 1 gb card vs a 2 gb card and there is absolutely NO performance benefit from the extra RAM.
For example, this chart shows a worst-case scenario, a game with one of the highest levels of memory usage at a resolution much higher than 1920 x 1200. There is no difference between the two 6950 cards, each with different amounts of memory. Save your money and get the 1 gb GTX 560 Ti.