I have a 27 inch, mid 2011 IMac. I am considering increasing the base memory of 4GBs to 12 GBs by using two 4GB modules. Is there a speed advantage of loading the 4GB modules in the top position and the two 2GB modules in the bottom position ?

No. The most important thing is to buy the correct RAM from reliable vendor's like OWC macsales.com or Crucial.com

If you are not sure I suggest you go here.

http://www.crucial.com

Then let them scan your computer and they will tell you exactly what you need and the cost. I suggest you Max your Ram. It will improve the responce of your computer.

Cheers

Don

No; just make sure you match the modules horizontally - so you can just leave the two OEM where they are and populate the the other two slots with the new RAM. And, as den.thed suggested, make sure you buy from a reliable vendor that offers a lifetime warranty.

dorothy197 wrote:

I have a 27 inch, mid 2011 IMac. I am considering increasing the base memory of 4GBs to 12 GBs ...

Macs can be very Fussy about RAM...

You can use these Links to check which RAM is suitable for your Mac...

http://eshop.macsales.com/shop/apple/memory/

http://www.crucial.com

It is Important to get the Correct and Matching RAM

Those sites also have videos on how to Install RAM should you need it...