The 7850 is really the only choice here. Maybe you should sell your 7770 to get more money for a 7870 because going from 7770 to a 7850 will only be a slight upgrade.
This is the correct answer. While the upgrade from a 7770 to a 7850 will be noticeable, if you could sell your 7770, then put the money made towards a 7870, you would see a much greater increase in performance.
At 1920x1080 the 7870 will perform very well at least until we see new titles (Crysis 3 )