Ok, if you're a hardcore gamer the GTX 680 is the best choice to go with as it's the strongest single GPU out there, followed by HD 7970 if you wanna experience a little difference of FPS in gaming.
Right now the best bang for buck is the HD 7970, you can't get a GTX 680 for $500 and the difference between the 2 cards is 7% @ stock speed, 2 % if you're going to OC the HD 7970.
EDIT: For some games it will be an overkill for that resolution, for some titles like Metro 2033 & Batman AC it's just a strong card can cope up with the demanding games. Something like GTX 580 or HD 7950 play all the games at that resolution very well.