Since it is 3 monitors then the GTX 560 Ti 2GB of Vram. Is the best way to go. you can alway OC your own cards and you can even buy the 2GB of Vram already OC'd.
If your Res is higher than 1920 x 1200 just on one monitor then you would bennafit from 2GB of Vram. But since you will be using 3 monitors it doesn't matter on the Res. I hope this helps and Good luck to you and your new cards.