what kind of out take or exhaust are you using
ive got this one and love it! http://www.rightpricehydroponics4u.co.uk/powerplant-aircooled-aerowing-reflector-no-cord-374-p.asp it was only £70 from my local hydro store, there no heat comes off and you can touch the glass and its barely warm,
i would do 6 m8 only because i know 100% a 6 inch 450 cfm fan is enough to cool a 600 light and most likely the room its in at the same time. (depending on size of room)