Let me be honest with you, you don't want to Crossfire the 6850. Get 1 more powerful card. I have a 6850, and in some games my CPU is becoming an issue. I have a C2Q Q8300, which is only 1 tier lower on the chart: http://www.tomshardware.com/reviews/gaming-cpu-review-overclock,3106-5.html
This is unless you OC, which could squeeze more performance out. Still, never crossfire 2 mid-range cards unless you already have 1.