I'm a bit curious here, maybe you can enlighten me for a bit. Tech-spec wise, the Early 2008 Mac Pro (3.1) should use a 800Mhz FB-DIMM DDR2 and older Mac Pros (1.1,2.1), are equipped with the 667Mhz FB-DIMM DDR2. I'm confused because my clearly 2008 3.1 MP is running the 667Mhz Rams. Is it backward compatible? Sorry for the silly question. But it seems that the Ram sockets accepts lower ram ratings because mine is currently loaded with the 667s instead of the 800s.