How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
A-Tech 8GB Kit (2x4GB) RAM for Apple MacBook Pro (Mid 2012), iMac (Late 2012, Early/Late 2013, Late 2014, Mid 2015), Mac Mini (Late 2012) | DDR3 1600MHz PC3-12800 SODIMM 204-Pin Memory Upgrade at Amazon.com
Apple iMac 27-inch (Late 2012) - Review 2012 - PCMag UK