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1440p and 3930k@ 4.6What resolution & system spec?
3 would be better, are you sure 2 is enough! ( this coming from a 670 user ).
1440p and 3930k@ 4.6
Two of these should suffice tbh :
http://www.overclockers.co.uk/showproduct.php?prodid=GX-201-MS&groupid=701&catid=1914&subcat=1750
However, as Kap said, the 780LTG will probably be the best 780 available.
Out of interest, what have your 7970 Lightnings been like? Good overclockers?