You are highly advised to read up on the latest FCAT reviews covering Crossfire setups.
http://www.pcper.com/reviews/Graphics-Cards/Frame-Rating-Dissected-Full-Details-Capture-based-Graphics-Performance-Test-3
Quote:
Where AMD has definite issues is with HD 7970s in CrossFire, and our Frame Rating testing is bringing that to light in a startling fashion. In half of our tested games, the pair of Radeon HD 7970s in CrossFire showed no appreciable measured or observed increase in performance compared to a single HD 7970. I cannot overstate that point more precisely: our results showed that in Battlefield 3, Crysis 3 and Sleeping Dogs, adding in another $400+ Radeon HD 7970 did nothing to improve your gaming experience, and in some cases made it worse by introducing frame time variances that lead to stutter."