How many CFM do I need?

My cabinet is 36"x18"x72" and i'm running a 150w HPS. My box is 27 cubic feet, what CFM fan should I get to vent out hot air. Right now I have a 4" computer fan as the vent and it worked fine with CFL's, but the HPS is too hot. Thanks!

I'd get the biggest you can afford. With those dimensions I'd go for one that can PULL >=400 cfm which would exchange the air in there @10x.

I dont think you need much at all with a 150 watt hps. The smaller fans should work.

400cfm is sounding overkill to me. I was looking at something around 100cfm, bathroom fan or the like. That would exchange the air 4 times a minute, does that sound adequate? Or something like the 6" inductor fan, booster fan for duct runs but has a free air flow rating of about 120cfm.

I think mine is only 150 cfm and i have 2 400 watt lights running.