It is more likely that you'll hit above 3GHz with E6600 than hit 3GHz with E4400. In average the E6600 are OC-ing to 3.4GHz, but that more depends of luck. Mine E6420 runs stable OC-ed to 3.2GHz and undervolted from 1.325 to 1.28125. But that is more like exception. The previous CPU was E6400 and needed 1.48v to run stable OC-ed to 3.2GHz.
So, check the OC potentials of your E6600 before you let it go. If it sux like mine E6400, than let it go and get another one. If not, keep it because the newer C2D's are clocking less than the older.