Ordered your 21.5" iMac with only 8GB?
OWC now offers a 16GB upgrade kit with all tools needed:
“We’re pleased to announce this complete, DIY memory upgrade kit for what was previously considered by many to be a non-upgradeable machine,” said Larry O’Connor, Founder and CEO, Other World Computing.
“It should be noted, though, in order to upgrade the memory, iMac owners must perform a highly complex screen and logic board removal. As such, we highly recommend professional installation of the kit.”
You can do the upgrade yourself, but in such cases the smart move is to pay OWC to have it done, unless your specifically enjoy this sort of thing.