I'm not expecting to use Ultra Settings, and I'm looking at the MSI Twin Frozr GTX cards.
The question is, at a resolution of 2560x1600, I would think the VRAM might be more important that jumping a card level.
So, what do you think?
GTX560 TI w 2G vram, MSI makes a nice card with 2G
GTX570 with 1.25G vram, or
GTX580 with 1.5G vram?
Cost is obviously a question as well, but a different concern.
For high resolutions, VRAM is important. So at those resolutions, the radeon HD 6970 is a good card that you may want to consider like iam2thecrowe said.
There are also 3GB versions of GTX 580s out there.
You could also attempt the HD6950 2GB and flash the BIOS to that of the HD6970's. Voids warranty 99.9% of the time, but it works and if you have a cooler like the Twin Frozr heat shouldn't be a problem as long as you don't push the OC. Very close to HD6970 performance for the price of an HD6950.