It depends how far you overclock it, and also what voltage is used to reach such an overclock.
For the common 3GHz @ stock volts overclock, it shouldn't run any hotter than 65W. If you increase the voltage however, power consumption shoots up pretty fast, and you could easily have a 100W chip on your hands.
For comparison a Q6600 probably maxes out at around 90W whereas an E8400 only uses about 30W under full load.