Its completely different: on a 21.5" you have to follow DIY instructions on somewhere like iFixit: cut the double-sided adhesive holding the screen on and partly disassemble the innards to get at the RAM slots, then you'll need a replacement set of adhesive tapes to put the screen back on. Not for the faint hearted - you might consider it to extend the life of an out-of-warranty iMac, but not on a brand-new one.
On the 27" 5k the RAM is
officially user-upgradeable -
you can find Apple's own instructions on their support site - just push a little button hidden by the power connector, a little door in the back of the case pops open and exposes just the RAM sockets and a little lever to release them. On an 8GB iMac there are two empty sockets - sweet spot is probably to get the 8GB model and add 2x8GB sticks to get 24GB. Crucial.com have a tool that points you to the correct RAM, and its the same Micron RAM that Apple use. You get a 24GB Mac for less than Apple's price for a 16GB one.
Only gotcha is that Crucial recommend a different, slightly slower RAM for the base i5 iMac 5k, as opposed to the faster i5 and i7 models - but their guide tool will tell you that.
Don't know why people are getting bogged down with "super-mega-X-ultra with go-faster stripes and fancy heatsink designed to look good in your illuminated plexiglass PC gaming rig and impress you friends by running Crysis at 150fps to their pathetic 149fps RAM"...
Thanks for all the advice from several of you in this thread. I have decided that I will buy a 27" iMac, but not right away. I will wait for Apple to release the 2018/19 27" iMac, then buy one. However, I will buy it with the standard 1 or 2TB hard drive instead of SSD, then replace it with a 1TB SSD from "macsales.com" (or Crucial) once the warranty is over. But I will order it with an 8GB RAM stick, and right after replace it with a 64GB RAM kit from "macsales."
I have been using a late 2009, 21.5" iMac with the following specifications:
-OS Yosemite version 10.10.5
-3.33GHz Intel Core Duo
-16GB RAM
-Graphics: ATI Redeon HD 4670 256GB
-1TB SATA hard drive
I have edited thousands of my photographs using this iMac, and while not very fast, it has done quite well for me using a standalone CS6, and some of the OneOne software, plus my favorite, NIK's software bundle. What I will do while I wait for the newer iMac is to replace the hard drive with a 2TB SSD.
I also have a 13 MacBook Pro that I have been using daily since 2011. I just use it at work for mundane tasks like checking or writing emails, writing documents using Nisus Writer Pro 2 (writing in different languages), Pages, and so on.
Specs:
-OS High Sierra
-13" MacBook Pro (early 2011)
-2.3GHz Intel Core i5
-16GB RAM
-Graphics: Intel HD 3000 512MB
-Hard drive: 320GB SATA
On this one I will replace the internal hard drive with a 500GB "machales.com" SSD for $195.00. The kit comes with the drive, an enclosure for it, the screwdrivers and other hardware needed to open the MacBook. I can use the enclosure with another hard drive or SSD. I can keep on using it as it is not, but the SSD kit is not a bad deal.