I never noticed a decrease in performance when using Aero. Rather the opposite in fact. When I disabled Aero after getting Vista, it always felt like my games were stuttering and nothing I did would improve it. No problems in full-screen mode, but comparing one full screen WoW to 4 windowed WoWs was never exactly apples and apples. Turned Aero back on, ran just like I expected it to. So I've just been ignoring all the "Disable Aero" advice out there, since having it on is obviously working better for me.