After 2.5 years the XBox 360 has sold (roughly) 19 Million systems, and if it can maintain this pace for another 4 years it would break 50 Million units sold. Personally, I have doubts that the XBox 360 will be able to continue this pace, and I also expect that Microsoft will release a new console in 2.5 years (which will hurt further XBox 360 sales) so I don't think it will come close to 50 Million units sold.