I have Win 7 64 bit running dual 6870s using CrossfireX. You will need to go to the AMD website and download the 11.12 Catalyst update which should be good for the entire 6800 series. If you have something other than Win 7 then just shop around till you find the right version.
Follow the link provided.
Now with 2 x 6870s and a six core processor, I still can not turn up the graphics all the way. Generally if I want a good flying experience, I turn off clouds and grass and put building and forest details at medium settings. Still looks good and gets rid of freeze ups.
http://sites.amd.com/us/game/downloa...n_win7-64.aspx
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