OWC 32GB (2 x 16GB) PC10600 DDR3 ECC-R 1333MHz DIMMs Memory Compatible with Mac Pro Early 2009 & Late 2010 'Nehalem' & 'Westmere' Systems and Early 2009 Xserve. (OWC85MP3S9M032K)
I have a 4GB 1333Mhz DDR3 and a 2GB 1066Mhz DDR3. Should I combine them to 6GB 1066Mhz DDR3? - Quora