War Thunder does not currently support CrossFireX and the game can have severe graphical glitches if CrossFireX is enabled. I have an AMD HD 6990 dual-GPU graphics card, and it's not possible to disable CrossFireX for multi-GPU cards using the AMD Catalyst Control Center, but the game was working fine for me until it suddenly started looking like this and was basically unplayable. I tried things like reinstalling the drivers and it didn't help, but then I discovered that it's possible to disable CrossFireX even for multi-GPU cards using a free 3rd party tool called RadeonPro. I created a profile for aces.exe using this tool and used the slider under Tweaks → CrossFireX Tweaks to turn it off for War Thunder and it solved the graphical glitches problem.
Hope this info helps someone, cheers.
Edit: RadeonPro is not required after all; going to AMD Catalyst Control Center → Gaming → 3D Application Settings allows you to create profiles for applications; just press Add …, find the path to aces.exe, and then there will be an option at the end to set CrossFireX to Disabled.
Edited by slikts, 18 December 2013 - 10:19 PM.