The page at http://us.battle.net/support/en/article/D3supportedvideo says "Diablo III does not support either SLI or Crossfire configurations". I have a radeon HD 6990 (dual GPU) card and catalyst 12.4 drivers. I'm not sure how to disable crossfire or check if crossfire is disabled. I can't easily find anything on google about it.
At the bottom of the "Gaming/3D Application Settings" page there's a section for AMD CrossFireX(tm) Mode for 3D Applications, but it has a large exclamation icon for warning saying "This setting applies only to user-defined application profiles." I can change it from the default "Default Mode" to "Disabled" (other modes are: "AFR Friendly, Optimize 1x1, Use AMD Pre-defined Profile"). However, this setting seems to get forgotten after reboot, and I'm not really sure it works or how to use application profiles.
I do experience severe framerate drops during fights in 4 player co-op. On max graphic settings, during heavy fights in Act 2, the framerate momentarily drops below even 10 FPS. Setting settings to lowest possible and disabling AA seems to help a little with it, but during very heavy fights the framerate still drops below 30 FPS. During normal "quiet" game play framerate stays constantly capped at 60 (VSync enabled).
I have a Phenom X6 1090 running 6 cores at 3.6GHz with 8GB of RAM running at 1600MHz and Windows 7. Catalyst 12.4 drivers for AMD Radeon HD 6990.
Also, is there a way to run some sort of timedemo to check performance? Having to gather up a 4 player party and get them in a fight every time I want to test a new driver/clock configuration is kinda cumbersome.
EDIT: looking at CPU usage graphs from task manager, it seems that the game isn't maxing out one core of the CPU.