i'm using 400 watt HPS for a 4'x4' area and 9 female plants... works fine

Rule of thumb is 25-50 watts per squar foot of grow space..... 3'x3' = 9 square feet

For 9 sq. feet i recommend 325-500 watts, but a HPS or MH will give you more light per watt, remember that! HPS bulbs are by far the most efficient, but Compact Flourescents are the easiest for a grower who is still living with parents.

Have fun