I would personally stick to one video card. I have a 660ti by msi. http://www.newegg.com/Product/Product.aspx?Item=N82E16814127697
I find that if your target resolution is above 1920x1080 you should be going with a 670 or higher. I think you would be fine with a 660ti either the MSI twinfrozer ones or the Asus DirectCU II would be the ones I'd suggest.
SLi uses a lot more power typically adds additional heat to your machine and doesn't always equate performance wise across every game/application. Only reason why I'd suggest one card 660ti or 670 in my opinion.