It should also be taken note that if you overclock the RAM, you are also overclocking (o/c) the CPU.
I'm not sure how it is on a Intel board, but I've seen on AMD boards in the BIOS " Host Clk + 33" (FSB + 33 MHz on RAM only) This would only o/c RAM and not the CPU. The CPU would say still run at 133 MHz, while RAM is running at 166 MHz. In that aspect for an AMD system, the benefits are really not going to be there.