You should be aware that CROSSFIRE simply does not work well. Also, if you are planning 2xHD7850 you'd be far better off putting that money towards a $400 GTX770. If a 770 is beyond your budget now, get a GTX760 (MSI has a great one at Newegg) for $250. If you have the cash you can get a second one later if needed.
While I'd probably recommend an i5-4670K instead, I'll go a head and recommend an AMD motherboard for you. However, I will recommend one that is for SLI or Crossfire.
Again, for multi-GPU the only cards working well are the new GTX700 series. You can Google that under "FCAT SLI" or similar.
You want an "AM3+" motherboard such as:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813130677
- supports CROSSFIRE and SLI
(I looked at several similar boards and there were NONE that didn't have several people with major issues so I couldn't point to one board with NO issues and say "that's the one". I guess that's what RMA is for in case you get a defective board.)