Hello all.. I have a late 2009 27" iMac.. 2.8GHz, Quad core i7. Right now I have 4GB of RAM that came preloaded onto the computer. 2 GB x 2. Excuse the stupid question, but I am not an expert on this, but if I add memory, do I have to stick with 2 GB in each of the 2 empty slots? Or can they be different. Example, 2GB x 2 and 4GB x 2 to equal 12GB total. Or do I need to go with all 4GB x 4?