I think Halo 3 performed well because Halo 1 and Halo 2 were popular games, this combined with lots of overhyping by the press and an unbelievable marketing campaign.
Surprisingly though this game did not seem to spike XBox 360 hardware sales as much as expected around its launch (there is no PC version currently available or even announced so far), the more expensive PS3 even outsold the 360 the months after its release. So it seems a lot of early 360 adopters were anticipating especially this game, shooters already performed relatively well (compared to great games such as Kameo).
I think the game did help spike XBox Live Gold sales, as the game is rather short and repetitive, IMO of pretty limited value without being able to play online. Certainly a great money maker for Microsoft.







