Jep that's true, if you overclock too much computer not even boot or it's unstable. I raised my 10MHz burts, and then i put my prosessor under the stress and i check my cpu temp at the same time. 
  
OC little (5-10Mhz)then drive 3D mark or something and check if it stable, if stable, raise a little again and test. 
There is really good CPU stress program at the internet too. Raises you CPU temp to max at few sec. 
  
How much you can OC is hard to say, mostly you need good CPU fan, i have Noctua-12P and when i overclocked my temps not even raised at all, but my memory is not overclocking more than 170FSB