MAC Pro RAM compatibility and performance

I was thinking of taking 8 GB (4 X 2 GB) of 667 mhz RAM out of my Mac Pro 1,1 and putting it in my Mac Pro 3,1. It would give me 16 GB of RAM instead of 12 GB. I understand that the 667 mhz RAM will underperform in the newer box.

Would the addition of 4GB of slower RAM be worth the switch?

via ehMac.ca http://ift.tt/1MBxye3