I currently have 12 GB. (2) 2GB and (2) 4GB. I am thinking of buying an 8GB and removing one of the 2GB. I keep reading comments about certain combinations not working or that they have to be the same size in each slot. Would this combination work? (1) 2GB (2) 4GB and (1) 8GB = 18GB If not what is the reasoning behind it so I have a better picture. Would it work if I got (2) 8GB and left (2) 4GB? Thanks for the help.