The problem is that this is an APU designed for a HTPC and such, so your CPU isn't that strong, could lead to a potential bottleneck so it's not only a power supply issue. I would say maybe a Radeon 7770 or 7850, but I wouldn't go for anything above that because of potential processor bottleneck. It will also depend on the type of games you will play, some titles are more CPU intensive and some others rely more on the GPU only.
well as of now, its onboard radeon HD 6410D, but i play games like DayZ (ArmA II mod, soon to be standalone), World of Tanks, RO2, and Aion. Arma is the most demanding game i play GPU/CPU wise