My father has just bought a new 2011 27" 2.7 GHz iMac and to say congratulations on buying an awesome new computer, I said I'd upgrade the RAM for him as he was going to pay for it from Apple! My question is, however: Apple's website says it has four slots, with the standard configuration of 2x 2GB. I've ordered 2x 4GB from Crucial to make 8GB as a replacement for the 4GB total, but surely if there's four slots could I not leave the 2x 2GB in place and then simply add the two new 4GB ones in the two extra empty slots so it had 12GB total? Or do they have to be installed in equal pairs? Many thanks!