Atig Reference Clock (mhz) & Sb Reference Clock (mhz)

[Hdhuntr01]

My BIOS allows me to change the settings of ATIG Reference Clock and SB Reference Clock however I can find no reference as to what role these two play in overclocking. I am looking for an explanation as to how these effect overclocking of my CPU and Memory. Can anyone shed some light on this or point me to a well written article I could read about their effects?