Officially, the "Mid-2007" and "Early 2008" models support a maximum of 4 GB of RAM, but they actually can support 6 GB. The "Early 2009" and "Mid-2009" models, however, both officially and unofficially support a maximum of 8 GB of RAM.
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I have two 4gb sticks I got from a laptop ( total of 8 GB ) Will it work? I want the imac to just use 6 gb ram but I don't got two 3 gb sticks. Do I have to use one of my 4gb sticks and use 1gb stick that came with the mac? Will the computer only use 3gb of the 4gb? I hope I could get 6 GB working instead of 5gb.