Many suggested that you need to render this 3D desktop all the time, even when in a game. They were right. The 3D desktop will run even when you play Half Life 2 or any other game.
We tried it and we even forced the game back to desktop and learned that game + Vista eats 1.35GB of memory but Aero glass works perfectly. It does not swap from the hard drive, it just works.
We tried to turn off Aero glass 3D interface to see if we can get some better performance. Well, we didn't. We got the same performance as with Aero turned on. ยต