How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
Timetec Hynix IC Apple DDR3 1333MHz PC3-10600 SODIMM Memory Upgrade for MacBook Pro 13/15/17 inch Early/Late 2011, iMac 21.5 inch Mid/Late 2010/2011, 27 inch Mid 2010/2011, Mac mini 5.1 & 5.2 Mid 2011: Amazon.de: Computer & Accessories