My I know the specific brand and model of HD6870 you have?
I am thinking of getting one to upgrade my Sapphire HD4870.
How vanilla is this 6870? Does it works by just using graphicsenabler=Y?
On 10.7.5 framebuffer does not patch works right out of the box
to 10.8.2, you need to patch
The card is not reference: 2 - DVI, 1 - HDMI, 1 - big DP
graphicsenabler = No
It works through the DSDT