Pretty sure it's the same setting as the divider, the only difference is that your bios tells you to ram speed instead of the ratio. This way you don't have to calculate the ram speed by yourself.
So yeah, if you are not overclocking choose the correct ram speed. If you are then it's probably just best to leave the divider alone since the ram speed will increase when you raise the fsb.
and yes, it could be the sensor. As long as it works fine I woudn't worry about it too much.