So I want 16GB with my new iMac. By default, 8 GB is installed as two 4 GB memory modules (and two slots are unoccupied). Does it make sense to add another 2x4 GB memory modules to have 16 GB total and all slots occupied (4x4GB) or should I rip out the existing 2x4 GB and buy 2x8 GB to get 16 GB total? Beside the costs are there any negative effects regarding processing speed etc? I guess it's also not a good idea to mix brands - what are the original ram units, samsung?