2 screens will not do anything to performance as you cannot do nvidia surround unless that is what you intend on doing?
Even so you need 2 GTX 560Ti's to even think of 3 monitor setups with nvidia.
1 560ti will do ultra but in DX11 with added textures expect around 25-40fps.
Well im not planning to do three screens, but two will do just fine. im only going to run with one card for now gonna add bits of pieces into my system once i get the money for it.