Watts per gallon is no longer used in marine aquariums. More important is the spectrum of the lights and the PAR of the light. Normal output fluorescents, VHO and Power compacts are all old technology and I would not wast money on buying any fixtures for those. On a tank your size a T5HO fixture would work great or a MH if you really want, or if you have money a LED system.
Here is an example of the Sunlight Supply wave T5HO fixture (a cheaper fixture) and
here is an example of a more expensive Tek fixture just to give you some ideas.