Right now I game on a 1080p monitor with my firstname.lastname@example.org and 5850 and I can max out every single game I play. I just ordered a 27'' Catleap and my 5850 will probably not be able to handle 2560x1440. I don't play a lot of the high-end games on the market right now, so anything that will suffice. I play Fifa 11 and Black Ops mainly; occasionally I play skyrim and fallout. Will be getting diablo 3 also if they release it in may.
I really don't want to spend more than 300. I was thinking of a 7850 or 7890. I haven't heard too much about the upcoming 6xx gpus, so im not sure if it would be worth waiting for those or not
Actually it's a good deal, because 7870=GTX 580 in terms of performance. But note GTX 580 will consume much more power and lack of new technology, such as PCI-E 3.0 (not that much difference BTW), DX11.1, Eyefinity, etc. It depends on you, you decide.