MVDDQ (VDDQ) is the 'chipset voltage' of a GPU (as compared to a motherboard).

Some claims that raising this improves the OC of both the core and the memory. Vddq had to be raised together with the Vddc to have any effect. Vddq was a 'big thing' back in the 9800 pro/xt days. Haven't seen it used since then. It had only a slight impact even on the 9800 cards.

Don't bother, I managed to kill two 9800 cards - both likely by playing with the vddq.