Can I run 533 MHz RAM in original Mac Pro? 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.
You can't use slower RAM. In fact I've had trouble with some manufacture's RAM even though it met the spec. You need to go to a site that guarantees any new RAM will work in a Macintosh.

I didn't think that the original Mac Pro could run Mountain Lion. I've got a Mac Pro 2.1, from 2007, that refuses to allow Mountain Lion to be installed. Did you hack this?