Only cards of the same base chip can be SLI'd/Crossfire'd.
The HD 7850/7870 have the same chip, but different clocks so they adjust to the most common lower speed for crossfire.
The GTX 670 and GTX 660 have different core configurations, and as such they can not be combined.
Again to recap, cards with the same Chip (core/shader count ect) can be combined even if they have different clock speeds.
And cards that have different chips can not be combined regardless.
in addition to that,,,,, they must have the same amount of memory and type