I think the Wii 2 will sell amazingly well for the first 6 months or so (on hype alone) but after that it depends on the quality of games the system has and what kind of media attention it is getting.
The PS4 should crush the PS3 in sales because I'm expecting the PS4 to be $299 at the most, just like the PS1 and PS2. And hopefully the PS4 will be BC with the PS3 which will help PS3 software sales into the life of the PS4 and help Sony make even more money.
The Next Xbox I expect to sell on par with the 360. Maybe slightly higher if lots of casuals by it because of Kinect.
Edit:
As for when they'll release, my best guess is:
- Wii 2 later this year, or Q1 2012
- 360 and PS3 Q4 of 2012, or if its not Q4 2012 it'll be Q3/Q4 2013.
I don't see MS and Sony letting the Wii 2 stay on market long without them having new consoles out there to challenge it.
