I got a pair of Sodimm Kingston RAM's at 2gb 800mhz a piece. I installed both at first, and the Macbook did not start up. The white light appeared, and I could feel the HD spinning, but there's nothing on the screen. Swapped the first slot from the left with my old 512mb, making it 512mb 667mhz + 2gb 800mhz, and it worked with Macbook recognizing 2.5gb @ 667mhz. Swapped the 2gb stick with the other 2gb, and it also works, meaning there's no faulty ram here. I bought the 800mhz so I could use it for my next Macbook, which I heard would be utilizing 800mhz RAM sticks. Quite sure the MB can handle 4gb (though recognizing at only 3.3gb), so is the mhz of the RAM the problem? I bought my Macbook in February. Thanks.