The only thing about the 2 watts per gallon rule, is it really is just a loose rule as there are many types of light and 2 watts of MHs aren't the same as 2 watts of PCs which aren't the same as 2 watts of T5s. Its a good places to start but it really depeneds on what kind of lights you are using. Also it depends on what kind of plants you want to go with as some require higher lighting.
