While I absolutely expect GTA IV to drive PS360 hardware sales, I think the bulk of those sales will come from people who had always planned on buying a PS360 and had never really considered a Wii.
For those who have a Wii, I doubt GTA IV is frequently going to get people to go out and get one of these machines. While GTA IV is exceptionally good, we have lots of other titles that appeal to the exact same demographic with the exact same tastes already.
Halo, Gears of War, Call of Duty -- all of these are also "cinematic" shooting games targetted at the 16-30 year old male demographic. All of them qualify as "very very good," and "very very popular."
Will it make a difference if they release a game that is very very good and very very very popular? I doubt it. It would have to be very very good, very very popular, and have a distinct style that appeals to a distinct demographic.
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