i have a 400w hps with 3 plants in a small ish wardrobe.have 1x 120mm PC fan suckin air in and 1x 120mm pc fan suckin air out .also in the ductin i have an 80mm pc fan suckin air out (dunno if this helps with the suction)
anyways .im in the uk an its winter so outside temps are low , but
those fans seem to do the job for me with my 400w hps .temps around 60-80 ish

http://boards.cannabis.com/showthread.php?t=46083