This is with a i7 970 @ 4.6 Ghz
Looking at the GPU score we notice only a 24% difference between a single 6990 OC'd (12046 GPU score) and TWO 7970s in crossfire. Thats only a 12% increase in performance per card (assuming 100% scalability) over a 6970. That is alot lower than you would expect given the 7970 is thought of as a card that closely matches a single 6990 in performance.
Source : Kitguru 7970 Crossfire Review
Later Today - Winrar benchmark for AMD FX 8150 @ 4.8 Ghz