It has massive performance boosts and its impossible to break a cpu doing it as there are to many safeguards. Bios is the most accurate temps but there are other programs when overclocking I use asus pc probe and speed fan running a prime 95 stress test for a couple of hours all free software. We will tell you
you will need a better heatsink to overclock the stock ones are rubbish and it should be a default that you get an aftermarket heatsink whether you overclock or not. Q9300 is newer the Q6600 is 2-3 years old now and the latest batches have been rubbish overclockers the Q9300 will get to 3.8ghz on air the Q6600 gets 3.2ghz. The Q9300 has more ondie memory so is faster from stock speeds aswell.
Not trying to make you broke just give you the best a cheaper option would be the asus xonar DX but you lose some functionality