i wouldn't focus on watts per gallon. you could have a bunch of standard t-12 bulbs dangling over your tank but it wouldn't give the same effect as a metal halide or t5ho. it's the brightness you should be concerned with. there needs to be enough light intensity reaching as far down as possible, and that's something standard fluorescents and compact fluorescents just can't accomplish.
for that kind of lighting (assuming pc lights) you could get away with lps and softies, but not sps.