I strongly recommend that you read the book "Gardening Indoors with H.I.D. Lights" by George Van Patten and Alyssa Bust. It provides a nice overview of everthing you would need to growing plants indoors.

In short, the main types of lights available are metal halide lights, high-pressure sodium lights, and conversion bulbs. Metal Halide lights are good for vegetative growth. High-pressure sodium lights are good for blooming and fruiting of plants. Conversion bulbs combine both aspects of HPS & MH bulbs and are supposed to provide good vegetative growth and blooming.

The basic HID system includes the following parts: ballast, lamp, socket, bulb, reflector. The ballast should be able to support the watts of any bulb. (A 400W CM bulb should be supported by a 400W ballast.)

The main wattages for bulbs (that I have noticed) are 250W, 400W, 600W, 1000W. In a 5 feet by 5 feet room, a 400-600W is said to be ideal.

I'll leave it to others to explain the best places to purchase supplies (I am interested in this as well). Others can also explain the different types of ballasts.

I'm new at this as well, so if someone has a picture of how their systems look, please post them.