i have that board and i do use the setting. heres the deal.
when i oc my 2.4 to 3.2 my games will crash after 10 mins. after i up the vddq i can oc to 3.5 and stable.
I think thats the juice for the ram on the videocard. my processor speed will go up more if the p4c800-deluxe would let me increase vddq beyond 2.8.
i disagree with ppl who says voltage kills cpu and/or whatever... as long as u have good cooling high voltage isn't a big deal.
in other words, its all about cooling, if u have enough cooling then up the voltage as high as u want!
conclusion, yes, vddq is there for a purpose, what does it exactly do? i don't have an exact answer for it BUT i know it does something since it let me oc 300mhz extra when i up it.