here's teh thing to check: 1) runs a game and turn the graphics down a couple notches. If FPS improves, your GPU is a bottleneck. If not, CPU is a bottleneck.
given you have 5750s in crossfire, and a 2.8 GHz processor, Its POSSIBLE that it's both. a 5750 is a pretty low-end item, and the 5xxx didn't have great scalability.
I'd say try to overclock the 720 to 3.5-3.8 GHz (and see if it's 4th core can unlock) -- what's your motherboard? then get a single HD 6870, as that's better than the 5750 crossfire.