Ensemble studios is hoping to redefine the way rts games are played on consoles and from what i've seen so far, the game is looking quite good. So it depends on how good the game actually is, and how close it feels to the other Halo games.
So if it is what they want it to be, and the Halo name is stil strong, i reckon it will sell about 3 million.
Definitely over a million though.