I Believe you are messing up your settings card side. Check your card settings, as everything should be in the middle of Quality and Performance.
I have a pair of 6870’s in CrossFire and I run the game at 6000 x 1080 (triple monitor setup) and I get nice stable 60 FPS. Both cards have 1GB of ram and it only uses about 50% of the GPUs.
I know that by running default settings in ati CCC, it runs smooth, but i am just curious of what card it would take if i wanted maxed CCC settings, because i find it looks better with max CCC settings, except my fps is low.
e.g. under gaming tab > 3d app settings in ati
you can set it up for indivual games or all games
try these setting lemme know if you can run them or what it would take to run them, if you have ati