I have an original Mac Pro with 2 GB of RAM in it. Running Mountain Lion on it is a bit painful - lots of paging. I happen to have 4 GB of PC2-4200 ECC FB-DIMMs kicking about. What I wanted to know is will the Mac run with the slower memory, or will it throw a hissy fit?
The Apple tech specs at http://support.apple.com/kb/SP30 say 667 MHz, but nothing about slower.