My E6600 is sitting quite happily on 3240 at 1.40V vcore with a load temp of ~43c under a Scythe Ninja.
My E6300 is sitting at 3150 on stock volts (1.325) with a load temp of ~38c under an Arctic 7. At the moment it is limited by the RAM but I will get faster RAM sometime and push it a bit more.
Both very good chips with the E6600 outperforming the E6300 somewhat due to the faster clock speed and extra cache but if I can get the E6300 to around 3400 with faster RAM, I would think there wouldn't be much in it.
I have had the E6600 running stable at 3400 but it needed 1.475V and the load temp was over 50c - I know that's not too bad but it isn't necessary for it to be that fast so no point it being that hot either (I prefer to keep all my rigs below 45c if I can - just personal preference).
Stan