Matte's right...what lighting you require depends on what you're planning to do. In fact, for a great many purposes, an on-camera light is not the best solution.
I recommend Jon Cooksey's "Digital Lighting Magic" instructional DVDs, available from Elite Video. They'll teach you a lot about OFF camera lighting.
The on-camera light is a compromise...it's quick, it's portable, it doesn't require anyone else to help set up...but it gives a "flat" lit appearance to people, can make them squint, and casts unfortunate shadows. For most purposes, a 25 to 50 watt light, with some sort of diffusion to soften the glare, will do. A dimmer is a very useful option, and some lights have one built in. You'll also need a power source...a battery pouch or belt.