I just upgraded my machine from 16G RAM to 24G and have 4x2G sticks of OWC RAM. This RAM costs $182 new from OWC and is specifically for the early 2008 Mac Pro, but will also work fine in the Older Mac Pro with 667 RAM. (The 800 RAM just runs at 667 in the older box.) Here is the OWC item, and note it carries a lifetime warranty: http://eshop.macsales.com/item/Other World Computing/64FB2MPK08GB/
Asking $135 OBO
Asking $135 OBO