Brian S. wrote:
You have an iMac with 8GB of RAM which is 2 4GB chips. The 27" iMac has 4 RAM slots so you can just leave the 2 - 4GB chips there and add 2 - 16GB chips and end up with 40GB of RAM just like the one I am trying on right now. It works great.
And as the other user said go to Other World Computing, macsales.com and they are great and even have live, human phone technical support if you need it.
Good luck
Brian
I did the same thing, bought two 16GB chips from OWC and put them in the two available slots. They have been working flawlessly in my late 2015 27 inch Mac since I bought it new many years ago.