Hey,
I have upgraded my early 2008 2,1 Xserve with 1x 2,8 GHz Quad Core Xeon with a second same CPU.
This was a few years ago in late 2013 and have never realy used it. Only a few hours.
In late 2016 the trouble started with an issue of CPU B FAN 2 IN spinning at 18 - 19k rpm all the time...