I followed these instructions to install Mountain Lion on my 2007 Mac Pro 2,1: http://www.jabbawok.net/?p=47

Note: For a Mac Pro 1,1 (2006), you may also want to update to the 2,1 firmware depending on what CPU you are using (have to create an account and login on these forums to download the file): http://forum.netkas.org/index.php/topic,1094.0.html (The 1,1 and 2,1 are identical hardware except for firmware)