How to upgrade a Mac: Swap in new RAM, graphics card, hard drive and more | Macworld
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
Apple iMac review (mid 2011) - The Verge
iMac (21,5 Zoll, Mitte 2011) - Technische Daten (DE)