3.0Ghz is pretty safe for that CPU. Of course, it can get much more especially with good cooling. Read the overclocking guide to start.
Q6600 has a good mutli so it will clock high and stay stable as long as u get the voltages right "they will be different for different setups". The main problem you will have to consider is the temperature; being a 65nm chip its gonna get hot pretty fast. Just start out a good base like 3.0ghz and see what ur temps are there + get your voltages right to stable it and then you can work from there.