You can't use the freshwater equation for lighting to determine lighting for a saltwater tank. You have to know the depth and length of your tank. For instance... if you were going the avenue of metal halide for a 55 gallon tank...you would need 2 bulbs as the tank is 4 feet long, and you would need at least 150watts per bulb (for standard 55 tank dimensions) if you wanted to keep all kinds of corals.
ps- there are many cheaper alternatives in lighting such as VHO and PC lighting, but my knowledge of them is very limited due to having metal halide bulbs myself. Perhaps someone else will chime in.
There are plenty of low-light critters you could put into your current tank. Sponges, gorgonians, feather dusters, sea-cucumbers, shrimp, crabs, etc..