Xbox 360 has an almost definite chance of hitting 50 million.
Let's look at the facts, shall we?
-19 million sold in 2.5 years. The generation is expected to last 6-7 years, and it'll probably sell for a year or two after that. A $199 Arcade (probably within the next several months) will boost sales quite a bit. A $199 Pro a year or two later would be a completely different story.
-They have yet to have a significant price drop in their biggest market.
-Despite last year having Halo 3, Mass Effect, Bioshock, and all manner of other gems, sales have increased SIGNIFICANTLY year-over-year so far.
So, with that, I tender my prediction:
X360: 50-65 million
PS3: 50-65 million
Wii: 150 million+