Hum,
are you sure?
On some systems, yes. I had to do it on mine.
Sure. You can reset it from the BIOS. But, if the RAM CAS is too far past where the system is stable, it may not stay up long enough to change back. (The system needs to be running to get to BIOS. ;-) )
Also noteworthy: A small change on bus speed, plus a small change on CAS speed can equal a HUGE change on the effective speed of your RAM, causing you to OC it to a point the system won't boot. Not that I did that and had to clear my CMOS... Nope.... (whistles innocently)