I say it will be huge but not as big as Halo or Halo2.  For one big reason Halo used to be the FPS king, used to.  Halo was the big bad FPS nothind could even come close to, but that was last gen.  

Right now Halo is not looking like all that.

Gears Bioshock URT3 CoD4 Killzone2 have stolen plenty of its hype, it will sell millions but not as much not as fast as before.

The hardcore will eat it up, but the other gamers they now have a choice Halo is no longer that one FPS you must play even if your not a FPs fan.

4 million in USA
1 million in all others.

Overall 5 million world wide. (pretty F'in good but less then the other two)

 I also think GTA4 will partially responcible by stealing plenty of the Core/Casual sales, and to a much smaller extent the fence sitters got a choice of get 360 with Halo or PS3 with URT3/Haze/Socom.

GTA4 won't steal any casual sales. Casual gamers don't like to roam around and beat the life out of people in a virtual city. Casuals will stick with the games that interest them, and cores will stick the games that interest them.

 Ohh yahh they do. 

Thats beauty of the game you dont have to play the missions, you can just go around killing and blowing stuff up (that what i do anyways) i never passed any of the GTA type games.  i always just screw around.

I hate to be the one to give you the bad news ...

Halo was never even close to being the best FPS on the market and still sold amazingly well because it was really marketed well. Halo 3 will probably not be even close to being the best FPS on the market and it will still sell amazingly well.

When you have

1. An established IP
2. A massive marketing budget
3. A moderately large userbase
4. A popular genre

it doesn't really matter how good your game is it will sell well.

(Please note, I'm not saying that Halo is a bad game I'm just saying it was never the best game; on top of that Halo 3 looks like it will be a very good game but I don't think that is going to impact sales much)

OK, right here we're confusing terms.

We're starting to mislabel, casual, non-gamer, mainstream, hardcore and serious gamers.

I say, casual are those who play short puzzle games that they find online (text twist, bejeweled, spider solitaire).

non-gamers are those new/reawakened gamers getting into a whole new apporaoch to gaming (Wii sports, animal crossing, brainage)

Mainstream gamers are the bulk of the PS2 crowd, into the usual bread and butter stuff that make up the traditional genres (Pro Evo, Madden, GT, GTA, FF), playing casually, but not 'casual' games

Hardcore gamers are the obsessive minority that get really heavily into their defined genre, mechanics and franchises (Halo, Counterstrike, Virtua Fighters, perhaps even WOW).

Serious gamers are those with esoteric, particular and expanded tastes who go for percieved quality (Katamari, Okami, Bioshock, Bushido Blade - my own personal vice).

Please feel free to fill in your own interpretations and examples.

 Sorry to break this to you but, with the sales it garnered last generation, it would be absolutely insane to think the game somehow missed the casual gamer. Casuals love games they can just pick up and play. GTA 3 and any game in the series thereafter offer that exact concept. Casuals eat that game up.

Halo sucks Bad..the story blows

and MC real name is John, what an Epic name Lawler

Many people who played PC FPS for years thought that Halo was heavily over-rated; Half-Life was a far more groundbreaking title that had a far better single player game and had the greatest online through mods (counterstrike).

It will sell millions, regardless of whether or not it has online co-op.