"can't over light a tank" are you serious?
I surely hope not.
The height of your tank and what is contained there-in, is all that matters.
And contrary to the above - yes, you most certainly can over light your tank.
My advice, be more concerned with kelvin over watts, why?
Simple - the higher the kelvin of any quality rated bulb the deeper it will illuminate the tank, all be it, with less visable "white" light, which for salt water critters, is usually better. (esp. with corals)
This also allows for those eye poping colors everyone wants so much.
Were it me, considering the fixture you have:
Coralvue 14k's or XM 15k's for the MH- dont waste your money on the other brands- they have no advantages, and two 420mn atinics, and two 460nm atinics.
The above allows for great diversity, and excelent PAR, which is what matters most.
Is this fixture a mogul based or de based MH? This also makes a big difference, caution with de bulbs- they have no coating like moguls do, they require the lens to be in place, otherwise, the radiation from these bulbs will kill your live stock.