i do have a P35 neo motherboard
but overclocking my cpu might also draw extra power from the psu, won't it?
yes it will
the basic processor consumption formulae is this :
CPUconsumption = constant * frequency * voltage * voltage
so this means that a higher frequency will multiply the consumption but not that much since quad cores @2.4Ghz don't really have a very high consumption..
though if you need to increase voltages that's a whole different matter, since as you can see in the formulae, any increase in voltage has a much more dramatic effect on consumption!
basically if you increase your quad core @3Ghz without touching the voltage, power consumption should be around what a real quad core of the same generation rated @3Ghz by intel would draw, so you might want to search the internet for what a QX6800 (2.93Ghz) would draw and increase the value a bit