PCworld article has a good coverage on this:
"Yes, you can overclock your RAM, too. But make sure you have matching sticks of RAM (same speed, manufacturer, and so on) before you start messing with the memory. It's much easier (and safer) to buy more memory than to overclock what you have. But if you've come this far, "easy" and "safe" probably don't mean much to you.
First, open up CPU-Z and flip to the ‘SPD' (Serial Presence Detect) tab to look at your machine's memory specs. Reboot into the BIOS menu and select the performance menu-most likely labeled ‘Performance' or ‘Configuration', depending on your motherboard manufacturer.
Enter the memory configuration menu (it was labeled ‘Memory Overrides' on our test bed), and disable the default memory profile. Adjust your memory multiplier by selecting a preset overclock option or by typing in the RAM clock speed. Use increments, one or two at a time, that match consumer-grade products (DDR3 RAM is typically sold in increments of 800-1066-1333-1600-1867-2133); save the changes; and exit the BIOS. Stress-test as usual, and consider running memory-testing software such as MemTest."