When you push up the megahertz levels of the core and memory, they'll do more work than they did before. This also means that they may generate more heat.

By removing some of the heat efficiently, it's possible to overclock a card even higher. A cooler card generally means a more stable card, which can possibly be overclocked further than in its stock state.

Your results will vary. Overclocking your video card is subject to the components used in building it. As a rule of thumb, some of the brand name cards tend to overclock better than the generic cards, but this is not always the case.