overclocking is running computer hardware at a higher spec then it was designed for if you know how to do it you can get better performance at the price of shorter component life.
as far as picking a card you need list what is in your comp cpu mb mem psu os...... and then say what you want it to do video editing, cad, games.... and of course what resolution you plan on doing said task at. Then someone will be able to give an educated guess of what you need.