My first responce is that you have about 40 sq. ft. of growing space, but you only have about 12 sq. ft. worth of lights (@ 50 watts/sq. ft.). 600 watts divided by 40 sq. ft. is only 15 watts/sq. ft. - that ain't much!
Secondly, get a ceiling fan speed control instead of a "dimmer switch". They look very similar, but are intended for two seperate purposes. They are usually sold in the same area of the store.