How Much CFM For AC Light?

How much cfm would be sufficent to cool a 600watt hps 6" air cooled. Before I purchase a fan it would be good to know. I see 6" Inline Duct Fans 150cfm for $49.99 and then there is 6" vortec style fans that pull way more cfm. I will be useing in a 4x4x7 area and the light will have its own fan and then I will have another fan to vent grow area since I want to use co2.

Bump, There isn't much data posted on this subject around any longer since OG left us. Sadly, I can't remember the formula to calculate the cfm needed per watt, length, bends and filters. Personally, Im sticking with 8" hoods and ducting going forward. Its much better IMO to oversize the diameter of ducting, fans and hoods. I prefer not running fans at max capacity because of noise, power usage and it also preserves the components of the fans, by using a solid state controller. Hopefully someone knowledgeable about this will chime in as I too am in need of a new fan.