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? Although, you could alway use it on your next Mac.I currently have 16 GB that came with my 2013 iMac. A TB3 SSD would be much faster than the USB SSD, but a lot more expensive. There are other external options for your iMac that are faster, but probably not worth the investment. Not a big deal if this was just to get your by for a few years til your next Mac or until you got around to installing it internally. This could lead to slower speeds later on and shorter lifespan for the SSD. The biggest downside of using USB versus internal SATA besides the slightly lower speed is that there is no TRIM support for USB drives on MacOS. Overpriced when you can get a SATA SSD and enclosure for much cheaper, and have the flexibility to use that SATA SSD internally if you wanted. I wouldn't do something like the T5 unless you already had one sitting around. It is cheap, faster than your HDD, and gives you the option to one day swap it with the HDD. That is one good thing about just using a SATA SSD and a USB enclosure. Click to expand.If you end up removing the internal HDD, you might as well just up a SATA SSD in it.

Imac 27 late 2013 ram upgrade