Watts per gallon is silly and I'm surprised people still recommend that. You can take 100 watts of T5ho, 100 watts of led, 100 watts of cf, 100 watts of metal halide and 100 watts of an incandescent light bulb etc etc etc and you will get a different amount of PAR/PUR and lumens per watt from one technology to the next.
If you are just doing softys you will be fine with your lighting. However like most new hobbyists, as you gain experience and knowledge, most likely you will want to branch out and try new things like Lps or even sps. It's nice to have a fixture that will grow with you. A good 4 lamp T5HO or LED system would be ideal.