i upgraded my single 6850 to dual 6850's in crossfire, big mistake that was. sold them both and got a single gtx660, and now im happy, my games run smooth.
Up until about 1yr ago the HD6800 series card had to many issues (stuttering etc.) with crossfire. But AMD solved these issues through driver updates and the HD6800 series cards work great in CF now.
A little over 1 year ago I would have warned you to stay away from CF with the 6800's but now (especially since you already have a 6850) I highly recommend a 2nd 6850 or even a 6870 for a Crossfire setup as a great upgrade!