Graphical artifacts could also be sign of a problem on your end. GPUs have a habit of doing strange things like this when they overheat or are on the verge of dieing.
I myself have a ATI Radeon 6990m which is a pretty similar notebook GPU to yours and I haven't seen this happen with Arma 3 Alpha.
Last thing I want to say: try updating your GPU driver. I'm currently running AMD Catalyst 13.1 which supports your card as well.