I was gaming on a 2560x1600 (1600p) monitor for 5 years with only crossfired radeon 4870s. Then I diagonoly graded to 2x 24" 1080p monitors on a radeon 7870 and had no trouble running 2560x1600 games on my 7870 before I went to dual monitors.
You don't need a 780/880 for 1440p.