On a single 6850 I've been able to OC up to 900/1200 on stock voltage in most games. Some games like ARMA 2 are not OC friendly for some reason.
On average I saw a 5-10 fps boost in most games.
On my crossfire 6850s the highest OC I've got stable is 929/1150 on stock volts.
I don't really see a increase of more than 2-7 fps in most applications when in crossfire (but I'm almost always 60+ fps anyway)
i have one hd6850 and you can go for sure to 900 / 1100 without change any voltages , just change the values on sapphire trixx and make test instability for 15 minutes ... and go to bf3 and see how much fps you got
instead of the sapphire trixx,i recommend the msi afterburner,,,overclock one at a time either core or memory clock...then benchmark to see any artifacts...if you dont see any artifacts,boost up the clocks higher..benchmark again to see if there is any atifacts..if you see artifacts,lower the clocks down...every card oc's differently..so you should take the hard way for getting the best oc possible...you will definitely get better performance in certain games by doing a good overclock///