It depends on the game. The Vram champ right now is Battlefield 3. Check out this series of benchmarks and note the diminishing returns for the GTX 580 1.5gb vs. the 7950 3gb as resolutions increase. 1.5 vs 3 gb is not a dealbreaker, but there is a difference.http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_680/9.html
Exactly what I was going to mention.
2gb is preferable for resolutions above 1920x1200. I could only see 3gb being needed for future games with 5760x1080 and high res texture packs with AA. There are only few games that will enjoy having more than 1gb at 1080p like BF3 and fallout/elderscrolls with 2k texture packs.