I don't think I would run anything higher than a 7770 unless you upgrade your power supply to 500 or 550. You might get lucky and a 7850 or 7870 would work, but you might also run in to problems. Many power supplies are rated at peak load, not continuous and also power supplies lose a little power as the capacitors age. A regular old cheap 450w that is 2 years old might really only be capable of providing 375w continuously.
6870 is probably not a great choice considering you can get a 7850 for about the same price and the 7850 uses a little less power and has a little bit higher performance.
at 1440*900 hd7770 is not necessary. neither is hd6870 nor the hd7850. at that resolution hd7750 is the perfect gpu. he said he wants to be able to play bf3 and cod:bo2 at medium/high i bet the hd7750 is more than capable of delivering playable frame rates. also with his current cpu anything above hd7750 does not make sense at low resolution. ok i have made my point that recommending expensive gpu for low resolution is plain giving wrong advice to OP.