You should use version 11.6 of the catalyst driver, it's faster and supports overclocking. You should also use either AMD APP SDK v 2.4 or 2.5. As for the issue it self, I believe it's related to having crossfire enabled. You can check to see if crossfire is enabled with the command:
You can disable it with:
aticonifg --adapter=all --cf=off
You'll have to restart your X server for the changes to take effect tho.