I don't think so, I think with the release of the final three well sold XBox sequels, Star Wars, GTA and Fable the install base should be at least on par with the original XBox this christmas. But then I expect the platform to fade away slowly, due to more fierce competition with the PS3.
How much the 360 will sell in the end depends upon how long Microsoft is going to support this platform, my guess would be 30 to 35 million. Europe and Japan are IMO as good as a lost fight for the long run, for the US I expect market saturation to hit the platform when the platform goes beyond the original XBox install base.