Sapphire HD 6970 2gb

What are total system specs? There are so many factors here that matter. First of all if your motherboard has 2 pci-e slots and if you use 2 graphic cards then the potential will be divided, so if you are using one graphic card slot it will provide x16 bandwidth but if u were to use 2 graphic cards then the performance will be divided to x8 x8 now this is all well and good if you are using ddr3 cards, but when it comes to gddr5 cards, card will not be giving it's full potential. Also your processor should be good enough to keep up with both the cards. I mean what is the point of double performance when it has to wait to be processed. Also Ram should be also enough.
 
Well if you already have 2 then i say you got much better muscle then majority out there. But if you were to upgrade for future i would say go with the R9 290X MSI Twin Frozr 4Gb Edition would be nice over the crossfire and would give best performance given the right processor and Ram.