I don't believe it will outsell Halo 3.  For a myriad of reasons.

- Historically, a huge selling successful title witnesses significantly less sales on it's sequel simply because in order to reach that many sales there was a lot of people who bought into the hype once and won't be fooled again the second time.  So while the 'fans' of the series will still buy the game in droves and there are a lot of fans for Halo, it won't hit that high mark that Halo 3 did.

Wrong

Just look at games in a similar position as Halo was on the 360.   (Gran Turismo 3 vs Gran Turismo 4).  Metal Gear 2 vs Metal Gear 3.  I'm sure there are plenty more examples that I'm missing.   I would like to see your list of games in the SAME position that Halo 3 / Reach sit in that have bucked the trend.

GT3 was bundled heavily, GT4 was not. In addition to this GT4 wasn't different in any significant way to GT3. The series has trended upwards for every single mainline release, this will be number 4.

Man over 4 million more bundles?  And how much different will Reach be than Halo 3?  Are they drastically changing the formula?  Or just re-tooling.  Looks pretty similar to me.

- Secondly,  Halo 3 was one of the first few epicly large titles for the Xbox 360 and also was Halo HD.. (Well actually it wasn't but you know what I mean).  For this reason, anyone associated with the Halo series that remotely enjoyed it went out of their way to go out and purchase this game.   This relates to my first point.  Less sales because some of the people who went out of their way to go out and purchase this game weren't happy with the direction of it, the gameplay in it , etc and won't get it because of it.     A good example for this IMHO would be God of War III.  I bet God of War III sells a good amount (For it's series) and most likely more than the other 2 God of Wars sold.  But I highly doubt if there was a God of War IV it would sell as much as III because the buzz / hype around it is lessened.   

Wrong

It's a fact of anything consumed. You don't sell 10 million copies without some amount of hype pushing those fringe purchasers over the edge.  For example,  Mario Galaxy vs Mario Galaxy 2.  I can promise you, I won't purchase MG2 because I didn't like the first one as much as I was told I would.

You're assuming most people didn't enjoy their purchase. #1 Live game and 100s of hours per person played proves you wrong.

Who said MOST people didn't enjoy their purchase?  If only 10% of people didn't enjoy their purchase,  then you're looking at over a million less sales.  #1 Live game means exactly what?  How many people play to this day?  Surely not 10,000,000 people..It proves nothing other than the people who love Halo, really LOVE the game.

Lose some people, gain some people, its pretty typical for any franchise really whether it gains more sales or loses them between installments. #1 Live means its the most played game to this day, millions of unique users per week. Considering the game is 2.5 years old its pretty astounding really and proves that the game is a genuine phenomenom which must not be underestimated.

- Thirdly,  It was a well established series.  So comparisons to Uncharted or a myriad of others don't really apply.   Uncharted 1 for example was always an underrated game and received no hype factor for it whatsoever.  Uncharted 2 got tons of media time and air time on top of having a core fanbase that loved the first one.

Irrelevant

It relates directly to what others were saying in terms of comparisons. New IPs don't factor into this because by the release of Halo 3 it certainly was not a new IP.  See Halo 1 vs Halo 2. 

Which has nothing to do with Halo: Reach

Nope.  But it does remove comparisons for Halo Reach with brand new franchises. (Something Halo Reach is not).

- The Halo name is probably at it's lowest point that it's been since the early days of the franchise.  And while Halo Reach can certainly go a great length towards putting it back on top,  the failures of ODST / Halo Wars do matter on the sense that they tarnish the Halo brand name and weren't up to traditional Halo standards.

It's common knowledge that the Halo brand name has been at a low point in terms of titles released.  Halo Wars / ODST were far less critically acclaimed, sold many less copies and frankly didn't live up to the Halo standard.

ODST is to Reach what GT5P is to GT5. Its a Halo 3 epilogue essentially. Call of Duty World at War also didn't sell to the Modern Warfare standard either, but Modern Warfare 2 sold even more.

Not even close.  There has yet to be a Full Gran Turismo released this generation.  When there is, it will sell huge because it's the first release this gen.    Modern Warfare / World at War are made by two different teams and are totally different games in a myriad of ways. 

ODST is not an indication of Halo Reach will sell.  It is a tarnish on the franchise though and it proves that the Halo brand name (While strong) is not nearly as strong as you are implying.   People will be more hesitant about Reach than they have been for any full release Halo thus far (Except the first one).  

Just as World at War and Call of Duty 3 were tarnishing the Call of Duty Franchise? I still fail to see how a game which came with a 1/3rd size single player in terms of content, a single multiplayer mode (with no matchmaking) and the Halo 3 multiplayer rehashed along with a Beta code which will sell 5M+ proves anything except that the franchise is strong. Everything else is pure speculation about how the userbase may or may not recieve the game.

I see around 7-8 million in sales.   Unless it's an absolutely revolutionary, once a gen type of game.

-  As has been proven, time and time again...Userbase doesn't have really much of an impact on sales.  Good games sell well regardless and have great legs.  So while the 360 user base has grown that really shouldn't impact sales one way or another. 

Wrong

There are plenty of examples to prove this.  GT3 vs GT4 (For example), Metal Gear 2 vs Metal Gear 3.   Please give me examples to the contrary.

GT3+4 sold more than GT1+2, also pretty much every Nintendo series had an increase in sales from the last to the previous generation. Zelda, Mario etc.

Gran Turismo the original game sold more than Gran Turismo 4.  Different user bases much?  Gran Turismo 1 released before Gran Turismo 2 and outsold a game with a larger userbase.  Gran Turismo 3 sold more copies than Gran Turismo 4 with a smaller userbase at it's disposal.

Yet Ocarina of Time on the pitiful N64 userbase clearly outsold Twilight Princess on the largest userbase Nintendo has had in forever.   As did Mario 64 vs Mario Galaxy. 

Quality of the game matters far more than Userbase.  If it's a good game, the sales will come.  All userbase determines is the possiblity for more sales.  It certainly doesn't guarantee anything.

All accounts put the quality and the experience significantly ahead of Halo 3 for both single and multiplayer. In any case the better comparison is with Modern Warfare than a racing game where the content includes a story line, unique multiplayer and the draw card of buying the game to play with your friends whereas a single player game doesn't have that same draw getting everyone along for week one purchases.

MW2 being a 2nd MW game on 360 and Halo Reach beint a 4TH Halo game on 360

or

Halo Reach being the 2nd FULL game on 360.

MW2 being the 5th full COD game on 360......

Halo Wars and Halo 3: ODST were priced at 60$, the price of a FULL NEW GAME.

Also, theres diffrence between COD WW2 games and COD MW games

Are you convieniently forgetting world at war?? and COD 3? and COD2 for that matter!!!