I've just bought a new iMac which came with 4GB of RAM but I want to upgrade it to at least 8GB and ideally 16GB (apparently the maximum it supports even though there are 4 slots each of which support 8GB DIMMS).

I wondering if there is any performance difference in using just 2 memory banks or all 4?

If I upgrade to 8GB should I buy 2 more 2GB DIMMS or replace the existing RAM and replace with 2x 4GB DIMMS?

If I upgrade to 16GB should I use 2x 8GB or 4x 4GB DIMMS?

Or does it not matter?