If you are gaming at 1920 x 1080 then both cards will do a good job. The main issue with the 660 is the 192 bit bus.
This reviews explains the issue. http://www.tomshardware.com/reviews/geforce-gtx-660-ti-memory-bandwidth-anti-aliasing,3283-11.html
Basically If you like to use a lot of AA features FXAA or MSAA etc, then you would be much better off from a flat out FPS performance point of view getting the 7870.
Please note that the FPS on the tests are not at any stage unplayable, its really only at 2560x1440 with 8 x MSAA that we start to see unplayable frame rates.
Personally I don't use that much AA and so it wouldn't bother me at all playing at 1920 x 1080 as I do.
Here is some SLI and Crossfire info http://www.tomshardware.com/reviews/geforce-gtx-660-geforce-gtx-650-benchmark,3297-14.html
Hope that all helps rather than confuses.