So my 2X4Gb RAM recently stopped working, yes, it just stopped working. So I bought some more RAM, but accidentally bought 1333Mhz instead of 1067Mhz. Now, I know it is possible to underclock them, but only manually (http://tadija.tumblr.com/post/3641585420/how-to-install-2x4gb-ddr3-1333mhz-in-macbook-pro-13 , http://www.youtube.com/watch?v=OJYDhOfRYEg) . When I put in 1X2Gb RAM and 1X4Gb RAM (6Gb) it doesn't boot up at all, the screen remains blank and makes a beep sound. However, others report that the screen lights up to the apple logo, but the loading bit stops and the OS loading process goes no further. I'm running Lion 10.7.4 if that changes anything.