I have a top model 2011 27" iMac excluding any RAM upgrades - it has 8gb. I was wondering where I can look online to max it out at 16gb (I assume 4x4gb sticks is the max as that is what Apple has told me) I don't want to pay extra for Apple to do it but one of the Genius Bar tech's showed me a site to buy the 8gb for I believe $140. This may be a strange question but can I replace the ram which is GDDR3 with GDDR5? Also, the tock ram has speeds of 1333MHz, can this be increased thru internal software tinkering on the Mac and do all the sticks have to have the same speeds? Any links to what is best or what a majority of you go to would be greatly appreciated!