I'm planning to pick up another 7970/280x for crossfire on a single monitor 2560x1440 or maybe 1600 in the future.
I've read that there are issues(microstuttering) regarding crossfire these past few months.
I play fps games mostly. In short, I'd like to avoid stutters and want smoother gameplay in 1440 resolution without reducing graphical settings in a game. Which is hard to achieve with a single 7970 on some games.
The latest drivers solved the micro stutter issues on single monitors up to 1600p so you should be in the clear. Apparently the new 290 and 290x don't have any stutter issues in crossfire for any resolution, just have to wait and see if they will release a software solution for their older hardware.