The size of the light you need depends on the square footage of space you are trying to light. If I remember correctly a 400W MH light will provide enough light to cover a 3X3 growing area. I would suggest that you get a switchable ballast, one that can fire either a MH or HPS. It will cost you a few bucks more but it's worth it. These lights get really hot, over a 1000 degrees or some such ridiculus number. In and enclosed grow room you definately need an exhaust fan on the hood as well as an exhaust fan from the actual grow room...there are some excellent web sites that will take you through each step of the process, just do a search cannijuana.