i have an athlon II x4 640 (Quad at standard 3.0Ghz)
if i want to overclock this, whats the best Ghz to wattage ratio to use?
obviously if i went to 3.3Ghz i'll have to upper the wattage but i'm not sure by how much?
As amuffin said it's voltage not wattage, and a 300Mhz OC might not require any voltage increase. I suggest you read some guides before attempting any overclocking though, you wouldn't want to damage your hardware.