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
8GB 2X 4GB Memory Compatible for Apple iMac Core i3 i5 i7 Mid 2010 DDR3 PC3-10600 204 pin 1333MHz SODIMM RAM (PARTS-QUICK Brand) at Amazon.com
Install memory in an iMac - Apple Support
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET
How to Upgrade Memory in a 21.5-inch or 27-inch iMac (Mid 2010) - YouTube