lights and grow boxes

im just wondering if what im doing is ok.
first of all i made a grow box out of 2x2's, about 5 feet high 5.5 feet across, and 1 foot 7 inches deep. i am going to cover the whole box in mylar and suspend lights by chain, should this be a good idea? (as i rent a basement it all has to be hidden in the closet. second of all how far should my 400 watt metal halides be from the plants (warm and cool). and how important is the ventalation as there isnt any source in the closet.

Mylar is good, as long as it's smooth without any creases, otherwise flat white paint is better.
Chains are standard equipment for hanging lights so good there.
Lights should be far enough away that you can put your hand just above the plants and it be warm, but not uncomfortable.
Ventilation is very important, especially if you have 800w of hid lighting in that small space. You need to get hot air out and away from the room, this should bring in cooler air from outside the closet, otherwise temps will skyrocket.
Good ideas are a carbon filter on the exhaust (for getting rid of the smell) and an oscillating desk fan inside the room to circulate air around the plants and promote strong stem growth by gently moving the plants.
Have a good look at your situation and decide if you can ventilate the closet somehow, you'll need to be able to make at least a 4" hole for ducting. Many people seem to think they can get by with just opening the door every once in a while, these people then go on to post threads with titles like 'I think I cooked my babies'.
The fact of the matter is those lights will need cooling, and plants won't like it if the temperature goes above 90 on a regular basis.