Everything I've read, suggests 3-5 watts per "gallon", Much of this depends on what livestock you are keeping. Your best bet is to think about what corals you plan on keeping and read up about thier light requirements before you go purchasing any lights. If you are going for just a few softies like the anemone and maybe some brains and leathers etc.. then the VHO's would be a good bet. But if want to keep the likes of acropora, etc... then you are going to have to consider metal halide lighting, maybe a couple of 175 watt halides with a couple of compacts this should bring your lighting to around 5 watts a "gallon" with is sufficient for these kind of corals.