No, supposedly MS will have 3GB on reserve for running their own things even while games are running. It has been suggested that Kinect might always be running, even during games that make no use of it.  Considering this is coming form devs using dev kits, it's all subject to change, but wasting so many resources would definitely diminish the quality of games. In the end it's software that makes it or breaks it (Gears and Halo made the xbox), but devs prefer the most powerful machines.