Seven days after its release, Halo 3 has just become one of the most important games ever, as the story of Master Chief’s adventures has reached more than $300 million in sales.

It seems like the $10 million invested by Microsoft in the Halo 3 marketing campaign had paid off and now the third part of the Halo trilogy has just entered in the history of gaming as one the fastest-selling video game.

Before its September 25 launch there were already one million pre-orders only in US.

Bill Gates said that Halo 3 is already a cultural phenomenon. “‘Halo 3’ embodies our vision for the future of entertainment, where some of the world’s greatest creative minds will deliver a new generation of interactive storytelling.”

Microsoft’s  Halo 3 brought the company more than $170 million in the first day of availability, surpassing by far its predecessor (Halo 2, with “only” $125 million) and this year’s Spiderman 3 ($67 million).

Microsoft also revealed that more than 2.7 million gamers have played “Halo 3” on Xbox LIVE in the first week, representing nearly one-third of the 7 million Xbox LIVE members worldwide. Within the first day of its launch, “Halo 3” players worldwide racked up more than 3.6 million hours of online gameplay, which increased more than elevenfold to 40 million hours by the end of the first week, representing more than 4,500 years of continuous gameplay.

Halo 3 is by Microsoft’s owned Bungies Studios. The action of the game revolves around Master Chief, a genetically altered, cybernetically enhanced Spartan supersoldier, who must race back to Earth to stop the Covenant, a fictional militaristic alliance of alien races, first introduced in the original Halo game. But it will be interesting to see if Halo 3 will be able to boost Xbox 360’s sales as Microsoft hopes. Released in November 2005 Xbox 360 sold so far 11, 6 million units, but Microsoft is facing a strong competition from Nintendo’s Wii.

Earlier this week, Nintendo of America President Reggie Fils-Aime said that he doubts that Halo 3 will help Microsoft selling its gaming platform.

“Will they sell a lot of software? Certainly. Will it sell hardware? I think it’s an open question. Why? Because I think that the “Halo 3″ consumer already has the hardware, because they’re playing “Bioshock” and “Crackdown” and a variety of games that are, in the end, quite similar: first-person shooter experience, multiplayer capable online. Tell me what’s new?” Fils-Aime said during an interview with Mercury News.