I've got a refurb 21.5" iMac 3.06GHz/4GB RAM/500GB HDD arriving soon. I believe it has two 2GB modules installed in two of the four memory slots, with two other slots unused. I have a few questions before I order more RAM for my machine. Is it advisable to have ALL the RAM modules be of identical brand and size? Can other brands of RAM be added to the two empty slots as long as they are of the same specifications? Do RAM modules need to be added in pairs or is single ok? Can the added RAM modules be of any size (1GB, 2GB, or 4GB) and still work fine with the factory installed 2GB modules? For example, would there be any performance problem by adding a single 4GB module to the existing two 2GB modules? Or would it be better to add two 2GB modules rather than a single 4GB module? Thanks, in advance, for the help.