the NB frequency is controlled by the CPU-NB multiplier. also, the CPU-NB is basically the integrated memory controller, not the NB. people say to keep the CPU-NB voltage the same as the CPU voltage and i've found it helps.
in AOD, i believe the NB VID refers to the CPU-NB voltage and the NB Volt refers to the actual NB voltage. i'm not too sure, it could be backwards, you'll have to check it out yoruself.