OC'ing is going beyond the typical supported specs. If you plan to do this then provide the "extra cooling" no matter what level of OC'ing. You're on your own risk as to what is best. No use to OC if it all coming crashing sooner or later with no added cooling. In other words, you'll find what works for ya.
Is there any possibility for over-clocking if I have only one fan in CPU with 512MB of RAM.