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Considering the cost (£120 for each card) a HD7950 would be a better choice.
+1
get a single card to reduce power consumption, noise, heat and driver problems
+2
Lower end cards are more prone to microstutter in crossfire than higher end cards, plus you have all the driver issues, heat, power consumption, and the fact that a 7950 would probably be faster than 7770 crossfire- especially overclocked given that the 7770 doesn't scale well with overclocking.
the 7770 is such a bad card... its on par with the 6850 which is cheaper anyways!
7770 IIRC was generally coming up second against the 6850.
Some games the 7770 can take the lead, Dirt 3 etc
unless my one good eye has failed me .. those benchmark you just posted dont include teh 6850
Why are the benchmarks not game benchmarks?