Atleast 10.5m....Could be more.

Remember: Halo 2, 3 years after release, sold well over 500,000 units this year....On a system that hasn't been in production for 2 years.

 Well that's actually a little deceptive.  I mean Halo 2 definetly had legs, but alot of those legs were attributed simply to the fact that Halo 2 had a new userbase to apply to, the 360.  Because of backwards compatibility this allowed people who bought a 360 and not an Xbox to own the game.  So I'd say that the majority of the copies bought after early to mid 2006 of Halo 2 were people who owned a 360 and not an Xbox.  However that many is I don't know but I think that should be taken into account.

 A sale is a sale. If your saying its not fair because of BC, well thats the point of BC. It also means the Xbox 360 is attracting new people. People that never owned the original Xbox and thats a great sign of growth. As long as the Xbox 360 keeps growing its userbase, its the same thing. 

I think somewhere between 7 and 8 million. Hollidays aren't going to change much about that, people who wanted Halo 3 had an early holiday already. There are tons of other great titles to get out there this holiday season to factor out the holidays too. And yeah Halo 2 didn't had competition either. Take Call of Duty 4 for instance, it won't outsell Halo 3, not by a large shot but in my opinion it has a far better online multiplayer than Halo 3. And by next year the competition has gotten only bigger and stronger!

 I know that's what I'm saying, its just now you have an extra userbase you need to take into account when doing attach rates. 

There is always competition! Imagine if Halo 3 launched in November. Would any xbox owner buy Cod4 or Assassins Creed? No way! It would have completely cannibalized all their sales. Thats why they didnt launch in Nov. Only a hand full of games can compete head on with Halo and those games are not it.

 But Grand Theft Auto IV is one of those that can compete with H3.  That is my main concern for long range sales for H3 in America.

"Halo 3 will be the best selling Halo game"

Nuh-uh, Halo 4 will beat it.

 Im doing attatch rates for Halo 3. 100% of Halo 3 owners are on the Xbox 360. Like I said, as long as the Xbox 360 keeps growing its userbase, the Halo 3 sales should keep increasing. Most 360 owners will probably end up getting Halo 3 sooner or later. Especially when it hits that $30 price. Halo is like the "mario" of Xbox 360. 

it will also have long legs in europe