I just built a new rig with a Phenom II x4 955 BE C3 stepping CPU. I have it overclocked to 4GHz on stock voltage. I was pretty amazed that I didn't have to adjust the voltage at all to keep it stable.
Idle temps are 40-41C, and under full load with Prime95 it goes no higher than 56-58C.
My case does have 3 120mm fans for intake, and 3 120mm for exhaust. The PS has its own fan for intake and exhaust.
Of course you can my friend got his to 4.8 stable but each is different i think average is stable 4.2-4.6
You should test to see if you get any benefit in data transfer from the over-clock not just a higher number.
There are many threads just for your processor.
Id find a happy place and stick with that don't want to burn anything up.
Heres 1 quick way I found to test Data Transfer Its on mycooltools.com
or u can run 3dmark