I suspect you might get higher frame rates, on paper, with two gtx660s in SLI, but I personally was never very impressed with SLI when I did it with two gtx560s, It just seemed to be more trouble than it was worth, and I personally was never able to get the 60-80%higher frame rate you would expect.
BUT, that is just MY experience, and that was several years ago, so perhaps things are a bit smoother now.