21.5in. iMac, which RAM should I configure with? I want it to last 4-5 years. I do everyday use with my computer.
16 is nice, but it depends on use. The problem is that these upgrades for longevity are often bad investments. Much of the time the amount spent aligns poorly with the extra time granted. There's a reasonable chance of hard drive failure within 5 years, so if you want to keep it that long, you should be comfortable with prying it open.
Check your page ins and outs on your current Mac. 8GB is plenty unless you are using some highly demanding software. As usual - it all depends on what you are doing with your Mac.