Hello, I am thinking of upgrading the RAM in my 2011 27 inch iMac to 12GB. I plan to keep using the stock 2x2GB RAM already installed in the computer. I'm also planning to order a 1x8GB stick from Crucial. My question is will 2x2GB+1x8GB sticks work? Can I use only three slots of the available four? I cannot find these answers anywhere, and it would be great to hear some advice!