I want to purchase certified memory for my good old dependable 2006 Mac Pro Dual-Core Intel Xeon, it needs: DDR2 667Mhz FB-DIMM PC2-5300 (not available from Apple now)... Or can someone suggest other brands...
I'd suggest taking a look here: http://eshop.macsales.com/shop/memory/Mac-Pro-Memory#667-memory. I used 16GB of their RAM in my Mac Pro 1,1 for years without difficulty. Their prices are good and they stand behind what they sell. I think you'd be better off not mixing the DIMM sizes.
I bought 2x2GB kit for less than OWC, and these are top tier memoryHow To Install and Remove Memory Mac Pro2x2GB FBDIMM DDR2 667MHz @ $39
Yes, that looks like the right type to get. See this page, which shows the kind NOT to get (bad heat spreaders). And I have seen a lot of these bad ones on the market, too: http://eshop.macsales.com/Reviews/Framework.cfm?page=Tips/fbdimms/fbdimms.html