What's your setup like?

I bought some fans from Home Depot. They were with the ventilation supplies like hoses (which you'll probably need) and adapters. I purchased the 6" fan which i believe is rated at 250CFM. It is set up on a box that's 2'x3'x4'6" with an enclosed 400w MH reflector. It keeps the box in the low 80's when the temperature outside the box is at room temperature. If it gets hot in the room, the temp might get as high as 90 in the box (a few inches below the glass of the reflector). I will probably upgrade at some point with a more powerful fan.

I would recommend getting more ventilation than you think you need. It's never bad to be on the safe side.