I didn't get to read all the responses on here, so forgive me if I say something that has already been said, but I recommend T5 lights if heat won't be an issue. I was using all T5's at one point but I have since made some of my tanks T-12's only (for plants that can't stand warm days). I like the Home Depot kind that sell for like 9$ because I can fit 3 fixtures (6 - 4ft. bulbs), on the grow racks we have come to know so well in this hobby (from Lowe's). Still, I have noticed the coloring on my plants has started to suffer from the decreased light. 4 bulb T5 lights are still better than 6 bulbs of T-12's. You could also use a metal halide light, or even a high pressure sodium lamp with a conversion bulb, but the heat output from these things is horrendous. I had a couple 600 watt fixtures and the heat is just unbearable.
Anywho, I have since decided to go big or go home, and will be starting up a 12x10x11 ft greenhouse fairly soon. I will also be writing up a little tutorial documenting my problems and responses along the way. With a fatty swamp cooler blasting away, I should get all the light I need and just the right temps .