Hmm, let me get this straight: Aero disables itself when you start a game? Does it give you an error stating: Low memory or similar? Does it enable itself after you stop gaming?
My setup does that with Fear2/L4D2 only, which doesn't bother me because after I stop gaming it enables itself back on automatically and I have the same amount of ram as you (6gb total [2gb x3]).
Unsure if you can asign more ram to Aero.