What do you have already? Only reason for asking is that if you have the kind of money to spend out on a new board, CPU, RAM etc you may be better looking at selling the 7850's and buying your new set up and sticking a new 1060 or 1070 in as a single card.
Crossfire is an absolute PITA at the best of times and when you factor in needing a better PSU, possible cooling issues, no 0 day profiles. The potential advantages are far outweighed.