Worst case scenarios:
Wii - 45m.
Nintendo's momentum dies in 2008 as third party developers find that they are losing sales again to Nintendo's own offerings. The "waggle" and graphics become unappealing as more people move to HD sets.
360 - 33m.
I don't see how the 360 can possibly do worse than the original Xbox. Add in the extra year of life and the fact that it STILL hasn't seen a price drop and ~10m more than the original 'box is the worst I see it doing.
Again, worst case. MGS and FF go to 360, price drops don't come quickly enough, Blu-Ray adoption rates stagnate.
More realistic but still pessimistic predictions:
Wii - 60m.
360 - 38m.
PS3 - 33m.
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