Some cards are OEM models and will only run half speed (or not at all) with AMD drivers. You need to use the driver from you card manufacturer.

Just got "OEM brown box" 7850 crossfire for Christmas and I was like WTF they run slower than my old 5830s!

After a while I figured out they were running 430/600 instead of 860/1200 and it was only fixed with the manufacturers driver disc.

The 6770 is not a good card, had one, it runs hot and dies if you get a hot summer. It too is an OEM card!