I have bought a new iMac 27 inch, and it has come with 2x4Gb RAM so 8Gb RAM, I want to upgrade it to 16 Gb RAM, is it possible that, only if i buy a single 8 gb ram and install it in the 3rd Rack will it work fine? or do i need to remove the previous installed RAM? and install a fresh 2x8Gb Ram...