I have a mid-2011 27" iMac with i7 processor. It came with 4 gigs of ram (2x2GB). I just bought 2x8GB of matched memory. I am wondering two things 1. is it good/bad/neutral to leave in the existing memory and add the 2x8 into the remaining two slots? 2. If you say it doesn't matter if it shows up as 20GB installed, but will only use 16GB of it, then should I put those 2x8 in the upper slots (the ones the factory 2x2 are presently occupying) and either leave out the 2x2 or put them in the bottom 2 slots. IOW does it use the upper two slots first during processing? Where can I find a speed test to see what difference it all makes?


So many questions, but I'll try to break them down:

  1. Leaving in the current 2x2GB that it has is good, it's better than 2x0GB.
  2. All 20GB will be recognized, the only reason you'd want to replace the current 2x2GB chips with 2x8GB is purely for the additional RAM.
  3. I recommend you pair the sticks up into the dual channel configuration for your iMac (as can be found online), so that one 8GB module and one 4GB module is on each of the two channels, enabling the use of dual channel memory speeds.
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    I have run my 2011 iMac in this configuration for several months now and there has been no issues. There is no benefit in swapping the order as long as they are dual channel. – Alain King Jun 27 '13 at 8:47

You only have to make sure that the same modules are installed in "dual channel" slots, to utilize the additional speed. More RAM the better so 20G won't hurt. Selecting either "upper" or "lower" slots should not make any difference.

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