I bought my aluminum imac when it was released in late 2007. I had a 1 gigabyte stick up until now, but I decided to upgrade to 2x2 gigabyte sticks. When I took out the 1 stick and placed the new ones in, my computer would sound like it turned on, but the system really did not boot up. I have tried many configurations to see what the problem with the RAM is, and all I could figure out is that the computer will only boot up if the original 1 gig stick is in one slot. I checked both sticks with this, and they seem to register on my mac when I check the "About This Mac" section. Basically, I get a black screen if I use only a 2 gig stick, or both 2 gig sticks. I can have a maximum of 3 gigs so far, but I really would like all 4 that I paid for. I'm not so sure that it's a problem with the ram, especially because it is recognized by the system, but it can't boot up the system, so I am simply dumbfounded. Anyone have an idea what the problem is? I just recently installed Snow Leopard prior to getting the RAM if that has any importance.