http://www.bghydro.com/BGH/static/articles/0606_vent.asp
Copy and paste the above link into your browser...some good advice on venilation, fans, filters, ect....i have a cfl grow running 15-23w with 2 260 cfm fans 1/intake and 1/exhaust. I also have a 260cfm oscillating fan (4'x2'x5' tent)...