you should go with single high end gpu which is always the better way.
the hd 6850 in cf causes some micro stuttering and theris some of driver issues and some games doesnt support cf setup so go with single high end gpu.
get this single hd 7850 it can play all games on 1080p with max settings with some aa it is somewhat better then gtx 560ti if you overclock it it can reach up to gtx 570 level its an very good overclocking card + this one is gigabbyte one its factory oced also you can oc it more its cooling is very nice and gives nice performance when compared to the price.
When you run two cards you will get micro-stuttering and it will draw way more power than one higher-end card.
The only reason you want to have 2 cards, is if you have a mid-level card and want to buy another because you are on a budget. Or if you are using 2 of the highest end cards because you are an uber cool gamer ^.^
I would suggest the 7850 and then overclock it a little. Really any top-10 tier card will be fine with D3. Look for ASUS, MSI, Sapphire.