How To: Upgrade 2017 5k iMac RAM to 64GB & Saved $840 | 2018 - YouTube
Install memory in an iMac - Apple Support
A-Tech 64GB (4x16GB) RAM for Apple iMac 2017 27 inch Retina 5K | DDR4 2400MHz SODIMM PC4-19200 2Rx8 1.2V 260-Pin SO-DIMM Memory Upgrade Kit at Amazon.com
Memory Upgrades for 2017 iMac 27-Inch
OWC Confirms New Low-End 27-inch iMac Can Be Fitted With 64GB RAM - MacRumors
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
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How to upgrade RAM in the (Mid 2017) 5K iMac and save up to ~$900 [Video] - 9to5Mac