a 400w metal halide would be ideal I think for 4-6 plants. just make sure you don't let it get to hot in there and expect to pay anywere from $20-35 extra a month in electricity. 400W is a little more than what a computer runs at (i gotta 350W power supply, but more typically you have about a 300W power supply on a computer) you may want to invest in some flouresent tubes too if you plan to make clones of your mother plant.