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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.
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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.
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