Original: Halo 2 is the best-selling first-generation Xbox game with 8.46 million copies sold by November 2008. As of September 25, 2007, Halo 2 was the fifth best-selling video game in the United States with 6.3 million copies sold, according to the NPD Group.
Simplified: Halo 2 has sold over 8 million copies in the US making it the best selling original Xbox game of all time.