iainmet: Have you tried disabling ATI Crossfire?
Crossfire is where the 2 ATI cards are running simultaneously (Nvidia cards call it SLI)
To do this open Catalyst Control Center, there should be a little ATI Icon next to your clock bottom right, if its not there it will be in programs or if you right click on an empty space of your desktop it will be in the menu that pops up, few options to access.
Now once you have opened this, go to the bottom of the options on the left and click Crossfire. There should be a checkbox on the next screen to enable / disable, try disabling it and then click apply or ok, now try starting the game again and see what happens.
I can't seem to find Crossfire.
You, someone else, and one of the mods on this forum mentioned that I have 2 cards but I do not. My computer only has one video card.
I'm assuming I would only be able to see Crossfire if I had 2 cards installed?