I'm inclined to agree with all of the others that for most folks 16GB is sufficient for a new iMac. That way if some other problem develops with the Mac, and Apple asks you to replace the original RAM as a way of ruling out your third-party RAM as the culprit, you can do so. Some people suggest that if you have to pull any Apple RAM to make room for higher-density RAM, you should save the Apple RAM in static-proof packaging. Apple does not consider the RAM on a current 21.5' iMac to be user-serviceable upgrade that iMac yourself, and you may well invalidate your warranty. On a 27' iMac, none, as long as you don't damage the rest of the Mac while installing the RAM. A current 21.5' iMac has only two RAM slots (hidden deep inside the machine) and is limited to 16 GB of RAM.

It is on a 27' iMac (an external access hatch provides access to four RAM slots). The answer is 'probably not', unless you are doing extremely complicated things (like stitching together lots of large, multi-layered photos in Photoshop Elements even then, 16 GB would probably be enough). Your recommendations where to purchase the RAM? Apple or third party? I suspect that if the warranty claim is not RAM related it's a non issue.

Some have advocated replacing the original RAM if you need warranty service, some say that Apple doesn't care. In my opinion,likely none, but I have no personal experience with a warranty claim. You may be able to replace it in less time than it would take me to explain the process I use mainly iPhoto and sometimes Aperture.

I have 32 in my 2012 27' iMac, but that's mostly because it was relatively cheap. Some are very vocal advocating that 32 is unnecessary. Doing large panoramas? Ton of layers in Photoshop files? If you have a problem with large files needing to use your swap file, performance will be improved. What is the benefit of adding another 16 gas of RAM? I currently have 16 GB that came with my 2013 iMac.

Imac 27 late 2013 ram replacement