We have a problem with definitions here. For example, you have an E5200 (2.5 GHz, 200MHz FSB X 12.5) with DDR2-800 RAM. I chose that because it makes the numbers easier. The BIOS reads the SPD chip and sets the memory clock to 800 MHz. Now, you raise the FSB freq to 300 MHz. Your memory clock will try to go to 1200 MHz. You are overclocking your memory.

Now, instead, you manually set the memclock to 400 MHz. and you raise the FSB to 300 MHz. Your memclock goes to 600 MHz. You are not overclocking your memory.

Now, instead, you have DDR2-533 memory installed with your E5200. Your FSB is 200 MHz and you set the memclock to 400 MHz. You raise the FSB to 300 MHz. Your memclock will try to go to 600 MHz. You are operating your memory outside of it's ratings, but you are still not OC'ing your memory.

Which are you trying to do?