Which memory speed for an early 2008 Mac Pro? I've got an early 2008 Mac Pro with 4 Gb of memory which I would like to upgrade, but I'm unsure of the memory speed. In my "About This Mac" dialog it says the memory speed should be 667 MHz, but online (even on Apple's own site), it says 800 MHz. Is it okay to get 800 MHz memory and mix the two, or should I hunt for 667 MHz memory? Or replace my existing 667 MHz memory with 800 MHz?
According to Everymac, your Mac Pro takes 800 MHz D DDR2 RAM. I'm not sure why your Mac is reporting 667 MHz, but try using the Crucial Memory Scanner and see which they recommend for your specific system.

As far as mixing RAM, it can be done, but it isn't really recommended. From a question on Super User:


  When different speed RAM modules are used, all of the RAM should clock to the speed of the slowest module. Many times, I've found that mixing RAM speeds sometimes causes system instability. I've had plenty of cases where RAM sticks that tested just fine alone wouldn't play well together. So I strongly recommend that all of the RAM installed in your computer be of the same speed.