when it comes to getting an Apple machine, always stick to standard memory config. Just accept the stock RAMs and source your upgrades elsewhere. I still remember Apple charges close to $600 to upgrade from the stock 4GB to 8GB on my Mac Mini... I bought my RAMs from SL Square for less than $100 =.="
iMac uses PC3-10600 1333MHz SODIMMs, the same memory modules as typical 2011 laptops. Installation video: iMac Memory Installation Video at OtherWorldComputing.com
Heck I just realize you can even dump 32GB of RAM into an iMac