the attached chart shows the lighting comparison for cfl and various hps sizes. for instance, a 600 watt hps provides 6,000 lumens per square ft at a distance of 2 ft to the canopy for a total grow area of 16 sq ft. you can also tell the light intensity directly under the 600 watt light is 95,000 lumens per square ft or about 10x the sun's intensity.
i did this to help answer the often asked "how much light" a little more detailed than "the more the better"