Must install in pairs.
.True Apple Qualified Memory for Mac Pro (2008) Quad-Core, 8-Core systems.0GB (4 x 4GB) OWC PC6400 DDR2 ECC 800MHz 240-Pin FB-DIMM Memory Upgrade Kit for Mac Pro (2008) *A 16.0GB (4 x 4GB) OWC PC6400 DDR2 ECC 800MHz 240-Pin FB-DIMM Memory Upgrade Kit for Mac Pro (2008) *Apple Qualified*.