It's been said, but it bears repeating; watts per gallon is a fairly useless rule, especially on very large and very small tanks. For instance, I have a 140 with 3.5 watts per gallon MH, and I have a 10 gallon with 9.6 watts per gallon PC. I grow SPS in the 140 fine, but I would not try them in the 10 gallon. If you can find PAR information, that's more useful, but the really useful measure is PAR/sq. meter (or sq. foot). It's the concentration of the intensity that counts.
As for your tank, if you definitely want SPS, you pretty much should go with metal halide, because they are the best way to get appropriate amounts of light over small spaces. If you want anything else, the Powerquad will work great (the need to alternate bulbs is not crucial, but the Current 2x 40 W will also work in place of the Powerquad). IMO/E, it's easier than you think to work around "less-than-adequate" light.