I heard for a computer to read RAM, all the sticks must be of equal value. I want to upgrade to another gig of ram, and was thinking of buying a 2gig stick and replacing one of my two 1gig sticks. Can I do that, have a 2gig and a 1gig stick? Or will I need to get 2 2gig sticks? I heard iMacs can only support 3gig of ram tops, but I'm not sure if you still need to get 2 2gig sticks for it to work.