The rule of thumb watts per gal
calculation only holds for lights that are full tank lenght (ie florcents). A MH
lamp will only light with any intensity a 2' square area so a 250W MH
lamp over a 3' tank will not give much intensity at the outside edges of the tank. In addition two 250W lamps on a 4' tank might give a total energy usage of 500W but the left side of the tank gets in effect NO light from the right side bulb and therefore any light sensitive items in that area is only getting light from the single 250W bulb.
You can not compare based on wattage a florecent and a MH
bulb due to the reasoning I gave above. In addition lets keep this topic on board with respect to the origonal question. Additional lighting questions merit their own topic so as to not hijack this thread.