It can also matter which slot you are putting them into. My older model Macbook has two RAM slots, both were filled with 2GB modules when I got it. I got a good deal on a single 4 GB module and figured I'd go ahead and use it. When I put it in the top slot, my Mac wouldn't boot, but after I swapped the two modules and put the new one in the bottom slot it came right up, recognized the new RAM, and correctly reported that I now had 6 GB.