Run it hot for a while. Basically pump some voltage to it without overclocking it, and just let it run warm for a few hours or longer. Maybe cut the fans in your case to get it warmer, jsut make sure you're not toasting it.
This allows for the RAM to get used to running hotter, and expands it. Basically speaking, it will "expand" the amount it can overclock, but this isn't always true. Sometimes it merely lets it begin to overclock alot easier, or sometimes it does nothing.
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Compaq Presario V5000 laptop