4GB RAM - Memory (PC3-10600) for Apple MacBook Pro 15 inch A1286 13 inch A1278, 17 inch A1297, Mac mini Unibody A1347, iMac 21.5 inch A1311, 27 inch A1312 (Early 2010 -Late 2011)MacWave
Install memory in an iMac - Apple Support
iMac Repair & Upgrade, Same Day Services
How to Upgrade Memory in a 21.5-inch or 27-inch iMac (Mid 2010) - YouTube
A-Tech 32GB (4x8GB) RAM for Apple iMac (Mid 2010 27 inch 4-Core, Mid 2011 21.5/27 inch) | DDR3 1333MHz PC3-10600 204-Pin SODIMM Memory Upgrade Kit at Amazon.com
NEMIX RAM 32GB (4x8GB Upgrade) DDR3-1333 PC3-10600 204-Pin SODIMM Memory for Apple MacBook Pro Early/Late 2011, iMac Mid 2010 27 inch 4-Core, iMac Mid 2011 21.5/27” , Mac Mini Mid 2011 at Amazon.com
iMac RAM upgrade: Here's an affordable way to improve multitasking - CNET