I have what may be a newbie question for all you portrait photographers.

What is the purpose of a modeling lamp, and how can it be used in measuring exposure for my flash units when doing studio portraits?

I have set up a main light, which is a canon 580EX. I am using a reflector for the fill light, and I dont know what I will be using for a kicker light just yet.

What would work for a modeling lamp without spending a lot of money, and what should I be looking for to help me gauge exposure of the subject correctly?

Thanks!