I see it hitting between 3 - 5 million. The Halo brand name along ensures that it'll do over a million, it would be the biggest surprise in gaming history if it didn't.
This will be pushed heavily and it wouldn't surprise me if it does over a million in its first week. Microsoft will be looking for this game to make the Xbox the system for RTS in the same way that Halo made it the console for FPS. This could really be big.