Unless the game shows to be a major disappointment just before release - which I don't believe will happen, even if the game is not that good, if the marketing is even half-competent - it will easily sell a million in its initial rush of 2-3 months, then another in the form of bundles, Platinums etc. The 5 million people who already bought Halo 3 didn't do it for competitive multiplayer, they bought it for the brand.
1