You would be better off to sell the 7850 and get a 670. First of all, it should be cheaper, assuming you sell it for a decent price. Now, the crossfire 7850 may have a bit more power overall, but the disadvantages is what causes me to say no to the option. You will have microstuttering issues that are common in any multi card setup. You will also experience poor performance on games that don't have decent drivers for crossfire. Some games won't support it at all, and others just won't scale well. Most AAA modern games will, however, scale pretty well for the most part.
Overall, the 670 will just be a much smoother experience with a lot less frustration. It will handle anything on the market at the highest settings at 1080p and get 60fps, except for possibly metro, which will cause almost any setup to cry.
The 7850 Nears 60fps on high settings on 1080p in Battefield 3, so I wouldn't go out and spend $400 on an upgrade. If you can crossfire you'd be much better off getting another 7850. Any game without a good profile would easily be hitting 60fps and the games with profiles will see major performance benefits. I haven't seen microstuttering so I can't comment on its effect.