Which memory speed for an early 2008 Mac Pro?
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.