Hi I got my iMac early 2011, (the model before the most recent one) and it has 4GBs of memory. Am I correct in saying that will be 2x 2GBs? And there are 4 RAM slots? I am considering a RAM upgrade and I can get 8GBs, 2x4GB sticks, can I just fill the 2 empty slots making 12GBs? Or is it recommended not to? Thanks.