Depends on your motherboard bios. Change the memory setting from auto to manual. Then if your default setting is 1066, use 1600; if it's 533, use 800. With some boards, the setting is half the actual ram speed. If 1333 is your top setting, look for a ram configurator setting that will overclock automatically, or increase your cpu speed by about 25%, and see how stable it is. For more help, list your make and model number. If it's an oem motherboard by dell, hp, etc, then you may have no options in the bios for changing the ram.