The overclocking tab shows the speeds of the card at different presets that the driver tells the card to run at. There are three main settings-3D, 2D, and Throt tIe. The speeds you set will be used instead of the default speeds for those profiles. Be sure your card isn't already running hot.

Any drastic change in speed could physically damage your card. Enter suitable speeds in the Extra fields. For this, you could use overclocking tools such as Rivatuner and PowerStrip. When you use them, you'll get a good idea of how far you can overclock your card and monitor its idle and load temperatures.