Unfortunately, the X5400 series of Xeons do not work in the 1,1 and 2,1 series of Mac Pros because of a chipset difference. The fastest processors you can put in a 2006 Mac Pro are the Xeon X5365 Quad Core 3.0GHz Xeons, which came in the 2,1 Mac Pro. Many users have had success with the X5355 Xeons which are also Quad Core, but run a 2.66GHz, and tend to be less expensive and more readily available than the X5365s with only a very slight loss in performance.