CPU pll is the part of cpu that runs FSB, so to speak. So, as you increase FSB, this voltage may need an increase in order for the cpu to continue functioning @ the overclocked FSB speeds.
Quads need more cpu pll than duals, from my experience, however the dualies I got all do 450 or higher without increase to CPU PLL. the one that stops @ 450 requires more voltage to go higher, and adjustment of this voltage allowed 25mhz FSB from stock, on a previous board that only offered a max of 1.7v for CPU PLL volts. I have not had a chance to throw that cpu in here yet.
The other cpu does 515mhz FSB with 1.5v CPU PLL, So i have yet to find a need to increase this voltage with this cpu yet.
I have not be able to determine if this truly is FSB-related or raw MHZ-related, but I'll be investigating this with this maxumus as it allows beter control of voltges than any other board I've got now.