Just bought a new iMac, wishing to purchase 64GB RAM to upgrade. Aside from leaving 2 DIMM slots open for upgrading to even higher RAM amounts in the future (find it difficult to believe it will ever be necessary) is there any practical difference between choosing to purchase 4 x 16GB modules versus 2 x 32GB modules. If I choose the latter (leaving 2 DIMM slots available) can I leave the factory installed 8GB in place, making the total 72GB?
And what about leaving the original 8GB in place? Is that possible? Wise?
Yes, please leave the original 8GB in their original slots and install the new RAM into the empty slots. The net result will be 72 GB of RAM which is WAY more than adequate for most users needs.
I would go with the latter as it offers more flexibility if you want to expand beyond 64 GB. When you purchase the RAM, please only use either Crucial.com or MacSales.com RAM as iMacs are very sensitive to using quality RAM. I know Macsales.com has kits specifically for your iMac. Below is the link to the OWC offering:
upgrade RAM - iMac 2019, 5K, 27 inch
