Let me begin by setting a couple parameters.
1) Don't say 3.42 million for PS3, don't say 8.17 million for Wii, and don't say 10.0 million for 360.
2) Assume at the very least that all three are continually sold through Dec 31, 2008 (no Dreamcast arguments).
3) Be realistic - Sony is going to end up selling 125 million PS2s in 7 years. Microsoft sold 24 million Xboxs in about 5 years, Nintendo will end up at around 22 million GCs in about 6 years, Dreamcast sold 9 million in about 18 months. For each console, the last year dragged down year sales averages significantly.
4) Last generation 180 million or so consoles were sold - the market is probably not going to shrink.
That said here is my take on the absolute lowest amount each console could realistically sell this generation:
Xbox 360 - 35 mllion
It is already at 10 million, it won't be killed as early as Xbox, it is selling about as well as Xbox at a higher price despite most Xbox sales coming at a cheaper price.
Nintendo Wii - 50 million
The launch was better worldwide than N64. Sales in Japan in 6 months (June 10 was the 6 month marker) are already 1/2 of lifetime N64 sales - a console which launched so strongly that 1/2 of it's sales in Japan happened by December 1997. Even if Wii declined like N64 in Japan, it would likely mean sales of 10 million+ since Wii will be at roughly 5 million by the end of 2007 in Japan. I expect Wii to do at least N64 numbers in the USA, and I think it will be Nintendo's most successful console in Europe, where sales are comparable to Japan on a weekly basis.
Playstation 3 - 40 million
Sony, almost no matter what, is going to do at least 7 million in Japan, and at least 20 million in Europe and the Americas on strength of brand alone. However, the huge userbase was based on people with no brand loyalty who simply thought PS2 was the best product (most games). In the worst case scenario for Sony, it turns out that 2/3 of their userbase is this kind of gamer, and abandons the Playstation brand. With Sony doing almost Gamecube numbers at $600, I would expect sales to increase fairly dramatically below the $400 barrier, meaning that the best years of the console will come later (2009/2010) as long as 3rd parties continue to support it.
People are difficult to govern because they have too much knowledge.
When there are more laws, there are more criminals.
- Lao Tzu