Crossfire is running two of the same ATI cards at once, effectively doubling performance (minus a fairly significant overhead, but still a huge performance boost). Only some motherboards have enough slots to fit two graphics cards, and if they can, then you use a special bridge cable thingy that clips onto both of them. For Nvidia cards, it's called SLI. What motherboard do you have?
He's asking if you want to run crossfire because two HD6850s perform better in crossfire, but worse individually, than two 5850s.