I can only speak for the newest iMacs, but the 27" models (regular and retina) have user-upgradeable RAM. The 21.5, however, is sealed and RAM is not user-replaceable. (I asked about this when I was at the Apple store two days ago).
The SSD is a module and could be replaced if OWC happens to carry that model. (Apple frequently changes the type of SSD they use internally, and it takes OWC etc around 6-9 months to catch up each time, I suspect the changes are deliberate) But the RAM is soldered down as is more frequently being the case.
