Don't Upgrade The RAM In Your New Apple iMac, Do This Instead
iMac Retina 5K: Memory upgrade (RAM)
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A-Tech 32GB Kit (4x8GB) RAM for Apple iMac (Late 2012, Late 2013, Late 2014, Mid 2015) | DDR3 1600MHz SODIMM PC3-12800 204-Pin SO-DIMM Max Memory Upgrade at Amazon.com
16Gb (2X8Gb) Memory RAM Compatible with Apple Imac "Core I5" 2.7 27-Inch (Mid-2011) By CMS Brand (A13) - Computer Memory Solutions
Amazon.com: OWC 32GB (2 x 16GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with Mac Mini (Late 2018), 27" and 21.5" iMac (Mid 2017), and Compatible PCs (OWC2400DDR4S32P) : Everything Else
How (And Why) To Install More RAM In Your iMac
Memory Upgrades for iMac w/ Retina 5K Display Late 2015