Mac Pro FB-Dimm Pairing Guide/Tests (w/mixed 1GB, 2GB and 4GB pairs)
SIX Core Apple Mac Pro 2009,2010,2012 CPU Upgrade kit X5690 Westmere 4,1 5,1 | eBay
Mac Pro 2009-2012 Repair (2009, 2010, 2011, 2012) - iFixit
OWC 6GB (2GB + 4GB) PC8500 DDR3 1066MHz SO-DIMMs Memory Compatible with Late 2008, Early 2009, Early 2010 MacBook, MacBook Pro Unibody Models, 2009/2010 Mac Mini, 2009 iMac Models
Memory Upgrades for 2009 Apple Mac Pro from OWC
8GB Memory Upgrade for Apple Mac Pro 2009-2012
Building up a Mac Pro 4,1 (Early 2009)...do I need ti update my RAM? : r/ macpro
Firmware hack converts 2009 Mac Pro to use faster RAM, CPUs | AppleInsider