Realistically I would say
100 to 120 Million for the Wii
40 to 60 Million for the PS3
30 to 50 Million for the XBox 360
The Wii has the potential to sell 20 Million or more systems in a given year and can probably maintain that pace for a maximum of about 4 or 5 years before I would expect sales to slow down; I would expect Nintendo to release their follow up system in 2011 and price it between $200 and $300 which will enable rapid adoption (and kill off the Wii quickly).
The XBox 360 and PS3 are both selling less than 10 Million systems per year; it would take either system quite a while (say 2 years) to build up enough momentium to sell between 10 and 20 Million units in a given year. As these systems start getting to a level where they can really take off a brand new system from their competition will be released which is at a very similar price and far more powerful.
