A-Tech 8GB RAM for Apple MacBook Pro (Early/Late 2011), iMac (Mid 2010 27 inch 4-Core, Mid 2011 21.5/27 inch), Mac Mini (Mid 2011) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Module at Amazon.com
Install memory in an iMac - Apple Support
RAM Upgrade for iMac Mid 2011 21.5-inch | MacRumors Forums
HOW TO: Upgrade The RAM to 32GB on a Mid-2011 21.5inch iMac - YouTube