it will have a lot to do with the quality of the games, we all know that halo 3 is going to be close to perfect, but we really dont know how good will galaxy be, if is a game that really changes the way platforms games are played it could sell a lot more than 10 millions with no much effort.
and in the case of halo everybody is forgetting the effect that gears of war 2 will have in the long term sales, 'cause this time there is a game that is just as good as halo in the online game. in the first xbox a lot of people continue buying halo and halo 2 during the lifespan of the console 'cause there was nothing as good as halo and this time wont be the case.
so my predictions will be.
galaxy: 6-15 millions depending of how good the game actually is
halo 3: 8 millions with at least 5 miilions by the end of the year just in NA
My definition of fanboy: An idiot that misses 2/3 of the fun.