Upgrade from 10.4.11 to 10.9 on a Mac Pro
There is only one available 2.66 GHZ Quad-Core Intel Xeon Mac Pro, which actually is powered by two 2.66 GHz dual core Intel Xeon 5150 processors: MacPro1,1

The maximum OS is Mac OS X 10.7.5, if the RAM was upgraded to at least 2 GB.