getting a 400watts hps will get you 57.14watts/sqft meanwhile a 250 watt hps will get you 35.71 watts/sqft.....over 30watts/sqft gets you good yield meanwhile over 50watts/sqft is considered to deliver an excellent yield....i'd say better buy a 400watt hps with a grolux bulb fitted.....you will not need to buy any materials such as mylar etc....the light will be more than adequate for an optimal yield...a yield that no 400watt of fluoros will ever match!