An AMD HD7970 is roughly equivalent to an AMD R9 280X, which as Belimawr points out would be a decent card for this resolution to test the water with.
But as your buying a new GPU anyway, for a single GPU setup at that resolution, i'd suggest either the AMD R9 290 or Nvidia GTX 780. These are good bang for buck compared to the price/performance difference from the 290X/780ti. And if you change GPU every couple of years, no need to spend a tremendous amount. I believe BF4 runs slightly better with AMD cards, although you will lose PhysX effects in the likes of Boarderlands etc... Performance is more important IMO.
Anyway, if i was in the market for a new single GPU at 1440p right here & now, the R9 290 or GTX 780 would be the ones i'd have my eye on.  I should also mention as a disclaimer, lol, that It is rumored that the new GTX 800 series is out later this year as well.
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Last edited by fatcobra; 03-01-2014 at 05:13 PM.