How does the memory divider work on 965 boards?

I have been messing around with a P5B over the weekend....the BIOS options let you pick AUTO/533/667/800 but I can't seem to find a pattern in what it leaves the mem speed at.

On top of that, I am also being told that the CPU can have an effect on what dividers are available........anyone with the info or a linky?

TIA