It has already match Halo 2's sales in the other countries and it's doing better than Halo 2 did in Japan. I'm afraid it'll take awhile for Halo 3's America sales to match Halo 2's since Halo 2's sales didn't fall down as fast as 3's did.

How high do you think Halo 3's lifetime sales will be?

8 million

It's a tough call. It's gotten 6 million shipped now but I question its longetivity compared to H2. Halo 2 was essentially the only huge game Xbox had at that time and really had no competition from other software. Halo 3 on the 360 doesn't have this same luxury. Nor will the game have huge legs in any place other than America.

So it's really going to depend on how the game does in America entering 2008.

If I had to take a guess I'd say 8 million is a good LTD number to go with right now but it could really go anywhere right now. It's hard to tell so early. I need holiday sales and early 2008 sales in America to really get a good guess.

Halo 2 had a nearly 20 million installed base when it was launched. Halo 3 has a nearly 50% attach rate and thats what primarily held back its sales and why it dropped so fast. It will have long legs though. In the end, I can see it at 10 million + sales.

At the end of it's lifetime I'm expecting 9,005,000.

Halo 2 has huge legs in America for sure, it has sold 470,376 so far this year.

 10 million is plausible but highly unlikely.  360 would need lifetime sales of well over 40 million for this to even be considered.  Not to mention I would assume most of that would come from America, meaning 8 million lifetime in America. Which I don't even thing many of the GTA and GT games from the PS2 did that in America, and it had an install base in America alone of 40 million.  So that's really a tough call but I won't rule out 10 million but its highly unlikely just for those reasons. 

9.5 million

 Your thinking about the sales in too much of a linear pattern.  It just doesn't work like that.  You need to think of outside patterns, trends, and of course remember that sales work in weird ways sometimes.  Not to mention the kind of game Halo is will sell in different ways then a game like Wii Play.