[header] How to buy ram for your imac [title] Buy no more than 16 gb of extra ram. [step] Imacs from 2011 , 2010, and late 2009 support a total of 16 gb of memory. [substeps] You can buy up to 4 different rams, or memory modules, for these imac models; however, they must not exceed a total of 16 gb in memory. 
[title] Buy ram with features that are compatible with your imac model. [step] Imac models from 2011 and 2010 use the same type of ram; whereas the late 2009 models use ram with slightly different features.