mhmm isn't that AMD GPU's scale better with Higher Resolution? While on 1920*1080 2x 680 is faster, on 5760*1080 2x AMD is faster ? depends on the game....
BTW; if you want to check out Resolutions higher than your Monitor suppors, aka testing 7680*1440 on a single Monitor, try out the "
Multi Monitor Performance Simulator"
BTW; i'd go with 3 7970. Cheaper... 3way CF also eliminates Microstutterting (at least its say to do so...) But then again you'd have to wait until driver supports 3way crossfire! sometimes takes quite a while!
On the other hand two 4gb 680 Watercooled OC'ed might also work. I'd get a OC version or OC myself if i'd go with just two GPU's.
i do run 2x HD7970 3gb atm, in 5760*1080 resolution and its not to much power!