Greetings
Disclaimer: I am no expert - new to this myself...
Having said that, I think I can offer some (very) general information on the lights. Typically, metal halide lights are considered to be the best in terms of shear light output per watt and their ability to support nearly any coral. There are, however, many factors that affect this and strong opinions on which is the "right" type of lighting. Suffice it to say that many different types and combinations of lighting will work and are preferred by people for different reasons (heat generated, electrical consumption, ease of installation. bulb life, etc.). Consequently, you can use lighting other than metal halide (e.g. "flourescent", T5, PC, etc.) and by combining a number of tubes, equal the total number of watts produced by a metal-halide system. Keep in mind, though, that watts-per-gallon is really not that useful as a measurement of suitability and matching the output wattage between two systems isn't necessarily an apples-to-apples comparison. There is no one right answer...
For a standard, 4-foot, 55-gallon tank, I think that most people would consider a total of 260 watts of PC lighting to be in the "medium" category, but given the right type of light, it should be enough to support many different coral species (recommending which corals is one area I will leave to the experts...). In short, it's not just about the number of watts. You need to also consider the depth of the tank and/or the placement of specimens within (bottom, middle, top), the color temperature and the photo period used, among other things.
For example, although my tank is also 4-feet long, it is a 90 gallon which means that it is taller and has more depth (front to back) than a 55 gallon tank. Consequently, a perfectly adequate lighting system for a 55 may not be sufficient for the 90 because specimens close to the bottom are farther away from the lights. It's surprising how much light is "filtered" by water. Even though you might not think so, the light intensity 12" below the surface is measurably more than the intensity 18" below the surface, given the same source light. This is one of many factors that affect coral health.
Again, I am not an expert, but I hope that these generalities help.
Take Care...