You manually have to edit MSI Afterburner to work fully. Trixx comes unlocked and it's what I used to OC my current Sapphire 6850.
Edit: Do not, I repeat, do not flash your BIOS. This only worked on the Radeon 6950 to 6970 because they were the exact same card except the 6950 had lower clock speeds. The 6850 & 6870 are different in clock speeds AND shader count. If you flash your 6850 you automatically void any warranty & can't RMA it and take a chance on bricking it.