Speaking from PC experience -- even if you buy 1333MHz memory, the BIOS controls the speed at which the memory operates...simply changing out the hardware won't do anything in and of itself. Now, I realize that Apples don't have BIOS, but I imagine the functionality is similar. In other words, the EFI (or something similar) will have to tell the system to run the memory at a higher clock speed. In other words, I highly doubt that simply changing the installed memory from 1066MHz to 1333MHz will do anything. Besides, the actual performance increase going from 1066MHz to 1333MHz would be negligible at best.