Memory Upgrade. As I couldn't get the computer to boot up with an old RAM 128mb + new 256mb, I put

Question

As I couldn't get the computer to boot up with an old RAM 128mb + new 256mb, I put the original 128mb back in but it would still not boot up. I have a feeling I should be putting in 2 x 256mb (for the upgrade). But I don't understand why the computer would not boot up when I put the originals back in.