Power consumption is equal to Capacitance times Frequency times voltage squared (P=C*F*V^2).
What voltage your graphics would need to overclock to what frequency will depend on the unit (how high you can go also thus depends on the highest voltages that you can use without killing the chip). This variable, IIRC, is the capacitance. So, since you can't know capacitance without testing it, you can't determine how much power you will need to consume to get a stable overclock unless you test to see what you can do at the top safe voltage and the frequency that you achieve at it. At that point, it's probably easier to determine your power consumption with a wattage measuring tool anyway.