Can I run 533 MHz RAM in original Mac Pro?
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?