Some BIOS's remember the configuration from last boot and then prompt the user to confirm at next boot the change to the memory. Others will run with the change without warning the user.
If your worried about losing your overclock settings then, I'd go into all the overclock settings and write down all the settings so that if a memory change does wipe it out back to normal native clock, you know what settings to put in to get back to where you were.
*** Only Big thing to worry about with this is that if your overclocking the FSB which overclocks your RAM you might find that the prior RAM you had worked fine with like a 10% overclock to the FSB, but the other RAM that is different from the initial RAM acts up under the overclock settings of what the prior RAM was happy with. Additionally if your messing with voltages (likely over voltage) to push components harder, some RAM is ok with this and other RAM get cooked to death. Additionally any undervoltage settings can cause instability as well if your trying to fine tune an overclock with an undervoltage to reduce heat and still have a fast reliable system.
With all overclocking and/or Over/Under Voltage settings, there is the risk of costly damage to hardware and data corruption, so be careful with it. And if messing with undervoltage settings definitely have the computer powered through a Battery Backup so that if there is a dip in line voltage to your home, it doesnt cause the system to be overly sensitive to a minor dip in the AC power to home or office as the battery backup would smooth out dips in power.
