You can run two monitors on one card no problem. You can run two monitors on a crossfire setup as well. If you are planning crossfire with two HD6850 cards they do not have to be the same model or manufacturer they will run at the speed of the lower card. HD6850 is a solid mid range offering with good performance at 1920 x 1080.
All looks good apart from the motherboard. That board has the second PCIe x16 slot running at X4 which is not as good for crossfire as this one http://www.gigabyte.us/products/product-page.aspx?pid=3... that will run crossfire at X8 X8.
Your board would be good for a single card setup so maybe HD6950?