Updated Catalyst to 11.4
I am able to get the 5760x1200 resolution when I run in windowed/fullscreen mode now.
But still unable to do it fullscreen. Crossfire has little benefit in windowed mode.

Looks like this is an AMD issue not a WoW issue.

I can't imagine that giving the executable file more than 2GB worth of memory is going to fix a video driver issue.
That memory issue was not related to not being able to select the resolution in the game.

I installed the AMD CAP application profiles updated 4/29 and the memory issue seems to be gone.

http://sites.amd.com/us/game/downloa...n_win7-64.aspx

The client running at 5760x1200 ultra settings, and a few addons runs right at 2GB.
Before the update I was crashing in Stormwind near the auction house.
Ran it for an hour today and no issues.

Just going to have to wait and see if 4.2 or the next Catalyst update fixes the fullscreen/crossfire DX11 issue.