take a look at this thread and you will understand better that the 660 is weaker than the the 970 .. the 660 in sli is about the performance of a gtx 780 and a 970 is a stronger card for the 780
I Will Go With The 970.
67.2 GPixel/s vs 19.6 GPixel/s (970 Have So Much Higher Pixel Rate)
And It Will Consume Less Power Than 2 660 And Is Newer So
970 Will Be Better.
I have 2 of them and was wanting to know if a single 970 would beat them on performance, thanks for the help in choosing , looks like I'm selling my 660's