You want at LEAST 5,000 lumens per square foot for a decent plant, although with mylar, you may get away with 3,000 - but dont expect brilliant yields, plus the buds will be fluffy and not that potent. Also it will take longer to veg and flower, and youll want to harvest the top of the plant first, then the bottom, unless you use some CFL's to light the bottom buds.

7,000 will give you some damn potent bud, but youll still want to harvest the top first then the bottom, unless you dont mind having some fluffy bud thats not as potent as the main colas. It will also be quite dense.

10,000 lumens is what you want ideally, IF you can control the heat, humidity etc. With 10,000 lumens per square foot, you can harvest the whole plant at once, with no noticeable difference in the potency of bottom buds - although they will be a little bit fluffier unless you get some extra lighting in for the bottom. Youre looking at rock hard buds, and it will be as potent as the packet describes. Also, you may be able to harvest a little bit earlier than normal, but only by a week at most.
Dont bother with anything more than 15,000 lumens per square foot, as the extra light is unusable by the plant. It can only absorb a certain amount, plus youll have to take care of the extra heat and electricity costs. For closet/stealth growers, either a 250w or a 400w MH/HPS will do the trick. Anything more than that and youll be wasting your money.
REMEMBER, THERE ARE OTHER FACTORS THAT DETERMINE POTENTCY AND YIELD - NUTES USED, SOIL, HYDROPONICS, PH OF THE WATER, HUMIDITY, TEMPERATURES, DRYING, CURING... THESE ARE ONLY SOME OF THE FACTORS!!!