400 watt HPS system

I recently bought a 400 watt hps system and am currently doing a test run in my closet (though this is by no means how I mean the final setup to be). Anyway, now that I have one of the two most expensive components of a grow box out of the way (genetics, lighting). I would like some advice on how much space a 400 watt hps light (with proper reflection off the walls/ballast) can light. I've been told 3x3. Thus, should I build a 3x3x3 box out of plywood and paint the interior of it white? Is this enough space that the light can still be properly ventilated with one or two computer fans and a bathroom vent? Also I'd like advice on where I can purchase a cheap deodorizer.

Lastly, is it safe for seedlings to start out under a 400 watt hps light? Or is this too intense. I have two babies who are growing about 2 feet under the light and one is doing just fine, the other, not so good. Their growth is incredibly slow at the moment (partially because I don't have proper reflection at the moment) and I thought it might be because they aren't close enough to the light, etc.

I have only done a one month run outdoors before (it got yanked) and thus don't know much about indoor growing yet, right now I"m just trying to get the components out of the way while testing how my bagseed reacts to the indoor condition. I'll post pics if the seedlings survive into veg.