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So 2 cards is the route to go over 1 card?
Running an overclocked 7970 at 1440p im able to fully max out all current games i play. (sleeping dogs/Battlefield 3/Hitman/Farcry 3/Black ops 2) Providing you don't mind running AA at x2 or below that is. Personally i dont think you need AA in a res this high with any game worth its salt. You can run SSAA on high in sleeping dogs though and maintain extremely high FPS. (thanks to the 7970's 32 compute units)
If money is no object id recommend one for for 1440p. If you're on a budget you can pickup a 7950 which should get you within 5-10% of the 7970 performance depending on the game and overclock its capable of.
If you're going to go down the multi card route and money is no object then you can't get better than 2x7970. A more sensible and cheaper option might be 2x7950 which you can get for a fair bit cheaper.
fully max out... If you turn down AA?
erm... that isn't fully max out then is it!
I'd say get a 7950 and add a second if the performance isn't up to your liking. Less than 5% difference clock for clock between the 7950 and 7970, so it is not worth spending the extra for a 7970.
Because you can really notice the difference at 1440p between 2xAA and 8xAA can't you.
The point is you don't have to turn down other eye candy, with a single 7970 at least. If you're happy running with lower levels of AA (x2 for example), or no AA at all (as it wont make any noticeable difference at 1440p) then a single 7970 is more than enough was my point.
Get a Gtx 680, look at my sig I've used all card setups on a single 1440/1600p screen and the Gtx is just so much smoother with every detail on max etc..
Hard to justify when a 7950 would be at least as fast as a 680 at 1440p and costs significantly less. If OP was adamant on going for Nvidia, the 680 is still hard to justify when the 670 is so close in performance and costs quite a bit less.