I have 2 6950s..both reference...one his and one asus...i went on techpowerup and searched their bios collection..they have unlocked bios of both cards..

ALWAYS use an unlocked 6950 bios rather than flashing it with a 6970 bios...the later will cause your card to run hotter, break the voltage control, and has know to cause damage due to the memory using different timings..

as for increasing the overclocking limits, i dont use the unofficial method in msi afterburner as then i cant clock down the cards at idle and i lose the power control 20+ option in CCC..

to get around the 840mhz limit, i used racerx's registry CCC hack and set my own upper and lower limits, that way i didnt have to enable 'unofficial overclocking'..google it