This is a question we have been asked a few times; to date Apple have used two different Intel sockets for their Mac Pro line. For the Mac Pro 1,1, 2,1 and 3,1 models (Mid-2006 to eatly-2008) it was Intel’s Socket 771, and for the 4,1 and 5,1 models (Early 2009, Mid 2010 an Mid 2012) it is Intel’s Socket 1366.
It is interesting to note that to date the platform has not been updated to the replacement of Socket 1366 – namely, Socket 2011. Given that Socket 2011 was released in 2011, it’s been quite a while since the Mac Pro has had a significant refresh.
And for those wondering, if you are bold, foolish or both you can replace and upgrade the CPUs yourself, with some caveats…