My 670 also does just fine at 1440p.. Sure, with Crysis 3 and Metro I have to turn down some minor options, but they aren't noticeable at all..
(Of course, everyone's experience in terms of 'enough performance' is different)
For better cards at a similar pricepoint I think you'd have to wait for AMD's 8000 series.. Otherwise, 7950 Crossfire would also be plenty of oomph. And with the 3GB framebuffer probably also for a while to come.
Of course you could also sell the 1440p display, but it's really all about what's worth what to you. The 1440p IPS panels are beautiful to look at, but the extra real estate will require more horsepower, and will require more expensive GPU's, or a quicker interval in upgrades, now and probably in the future as well. If that's worth it to you, stick with it. If not, go with a 1080p display, and sell the 1440p one. I'm sure you can easily find someone who would like to take it off your hands