Lets look at the facts.
It was number 1 most anticipated game of 2004 on the Spike TV Video Game awards beating out popular PS2 games.
Its been number 1 for the last 2 years in EGM for top 10 games I am waiting for.
Its number 1 on console preorders at amazon.com, ebgames.com, and gamestop.com
In the EGM top 10 games of all time on MTV Halo 2 was the number 1 reason 2004 will rock, with games like GT4, MGS3, etc, etc, all coming in behind it, and Halo as the number 2 best game of all time only being beat by the ENTIRE Zelda franchise.
On CNN it was reported to be the most anticipated game of all time. ON actual TV they said this.
On MTVS "10 coolest gadgets of 2004" Halo 2 was actually mentioned as one of the things you "must own in 2004", infact, it was even the only game that was mentioned.
So far no game, not even GTA SA, has managed all of this hype in the mainstream. If all of these things arent an indication that Halo 2 will be one of the biggest selling games of all time then I dont know what is.