Use MSI afterburner.
1. Increase Memory and GPU frequency slightly until you get serious problems. then increase voltage (if you get slight tearing try changing the screen resolution to another and back)
2. When your problems arise...
However many ports you have on the first card is the max while cross fired. And just a warning you wont be able to max every game in eyefinity. Even my 2 7970s at 1150 MHz cannot run BioShock infinite or borderlands 2 totally maxed....