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Do you have the 2010 iMac what uses 1333MHz RAM? If so, get the PC3-10600 as otherwise the stock modules will run at 1066MHz if you add 1066MHz RAM. You may not notice any difference but at least I don't want to weaken my machine to save few $
 
I do have the 1333 MHz ram in my iMac. I guess I'll go with the PC3-10600. Thanks for the advice Hellhammer.
 
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