Good choice on processor. Hopefully you have a cpu cooler for over clocking it.
Good choice on HDD. Western Digital, reliable.
Good choice on PSU. AX series, trustworthy.
Good choice on Motherboard. extreme4, all oc needs and crossfire.
Okay choice on RAM.
Weird choice on 6850. If you are buying 2 of those, just get a 7850 / 7870.
I think the reason crossfire is not a good idea for you is because the board cuts the power if you have 2 video cards running. what newegg says is "PCI Express 3.0 x16 -- 2 (x16/0 or x8/x8)" which I take to mean if you have one card will run at x16, if you have to they will be underpowered. or underperform. I'm not entirely sure what the x16 means (I did dremel and x16 into an x8 once, so I kind of get that it's scalable).
If it's really important to do crossfire, get a board that says something like x16/x16 or 2(x16). otherwise, as Ichrisk says above, get the single card.