How to Upgrade Memory in a Late 2009 21.5-inch or 27-inch iMac - YouTube
CMS 16GB (4X4GB) DDR3 10600 1333MHZ Non ECC SODIMM Memory Ram Upgrade Compatible with Apple® iMac Core I3 3.06 21.5-Inch (Mid-2010) - A28 at Amazon.com
Install memory in an iMac - Apple Support
Apple iMac 2010 8GB DDR3 1333MHz PC3-10600 204-Pin SoDIMM
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
Install memory in an iMac - Apple Support
A-Tech for Apple 4GB Kit (2X 2GB) DDR3 1067MHz / 1066MHz PC3-8500 SODIMM Memory RAM
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