While chatting with GameDaily BIZ one of the employees announced that Gears Of War 2 already sold over 4 million copies. It only took 2 months to reach this result. The sequel definitely beats original Gears Of War by sales. Gears of War 1 only sold 2 million copies in the same time frame.
If we were to total Gears of War and Gears of War 2, then the sum is around 9 million boxes. That is excluding PC-version since it wasn't as successful as console version. These statistics give us a good prediction of Gears of War 3 which we'll probably see before Xbox 360 dies out.