https://forums.macrumors.com/threads/2017-27-inch-retina-5k-imac-128gb-ram-upgrade.2181295/ If you are looking to upgrade your 2017 iMac 27" 5K's you can up it to 128GB of RAM. Same RAM modules used by 2019 iMac 27" 5K. https://www.amazon.com/Samsung-2666MHz-Memory-Computers-M471A4G43MB1/dp/B07N124XDS/ $120.75 32GB DDR4 2666MHz (1 x 32GB) $241.50 64GB DDR4 2666MHz (2 x 32GB) $483.00 128GB DDR4 2666MHz (4 x 32GB) https://www.samsung.com/semiconductor/dram/module/M471A4G43MB1-CTD/ Personally I'd only go up to 2x32GB + 2x4GB for 72GB of RAM.
^Something in me says I'd be better with a 4 X 16GB config (64GB) - uniform... rather than a non-uniform 72GB config. - assuming I don't wanna throw some stash to max out
