You want to upgrade from a 560 Ti 1gb to a 560 Ti 2gb? That would not at all benefit you, IMO.
2gb is good, I've hit the 1gb ceiling in multiple games. However a single 560 Ti won't really have the horsepower to push all the graphics features that need VRAM, besides textures.
If you want more performance I'd say get another 1gb 560 Ti. If you want an upgrade, check out the 7870 2gb.