This has probably been asked but was not able to find using the search. My macbook has 6GB of ram upgraded from 2GB When I upgraded I did not notice I put in 2GB of 1066MHz and 4GB of 1600MHz. So I found 4GB of 1600MHz in a dead laptop in our scrap/parts pile at work. I installed and only get 3 beeps. So it is too fast for the "bios" or what ever apple use to use it. The 1600MHz must be clocking down to 1066MHz as they are mixed. Is it possible to clock down without mixing? I doubt this very much though.