I'm thinking about purchasing a 21.5" iMac. The base model comes with 8GB of RAM, and the RAM is not user-upgradeable down the road. Is 8GB enough memory or should I upgrade to 16GB?

What do you want to do with this iMac?

I like to play flash games and listen to internet radio. Occasionally, I might want to stream a movie.

Those are all memory-intensive tasks. You may consider 16GB, but you could probably get by with 8.

For what you list, 8GB should be adequate. I ran with 4GB without any issue for over a year in my iMac i7 before I added RAM, primarily for running Win 7 via Bootcamp.

The tasks that need a lot of RAM are typically video editing/production, heavy-duty photo manipulation (not iPhoto or PS Elements), and professional page layout like Adobe Creative Suite. If you think that, down the road, you may want to delve into those fileds with this computer, by all means get the extra RAM.

I like to play flash games

I didn't try a lot of Flash games while I still had 4GB RAM but those I tried ran without issue and without causing a non-trivial number of page-outs

My bad.

Since you cannot upgrade at all later, get the new iMac with 16 GBs of RAM.

Is Flash not a memory-sucker?

Things posted fast! See my edit made while we were all typing. Yes, Flash can hog but, functionally, my i7 handled Flash games and vids without problem when I had only 4GB RAM.

Ah, no worries. I'm not sure how many "Flash things" the OP intends to have running at one time.

It's really awful Apple is forcing this decision on people wanting to buy the new 21.5 at the time of purchase. I don't know what the price difference is, but if it's not that terrible I'd get the 16GB because you never know what you might need it for later.

When I bought my G3 over ten years ago, I got an extra 256MB. That's megabytes!

You can install more RAM to the Late 2012 iMac, but it's difficult > http://eshop.macsales.com/shop/memory/iMac/2012_21.5/DDR3L

Also, see that it could void the warranty if you don't do it correctly

8GB are enough.

You should notice, that the new iMac is loud (fan problems?). C3 idle Issue!....etc. etc. etc. many many Issues! MacOS feels slow, because of 5400rpm HDD. Windows 7 works great. I do not think that Apple will release Boot Camp with Windows 8 Support soon.

Do not buy it!

You are welcome