If it were me i'd run a 600 dimable ballast with the largest hood you can fit , dual 400's would work too but if you ever decide to set up a larger grow your stuck with 400 watt dust collectors .
Im very partial to light systems from
WWW.HTGSUPPLY.COM , you can pick up 600 watt systems complete with air cooled hood , dimable ballast & bulb , plug & play for $220 & they make good shit , ballasts come with 5 yr no questions asked return policy & their bulbs last as long as big name bulbs .
I had 24 of htg's 600 hps going 12/12 for 6 yrs now & 2 other 600's going 24/7 , out of all of them ive only had 1 ballast go bad a year past warranty & they still sent me a new ballast free .