I assume you are switch from fish-only to reef. If you only have fish, there is no need for the extra wattage. If you are switching, the depth of your tank is what is important. For those who use metal halide, the general rule of thumb would be 150watts for 12 inches or less, 250watts 13-24inches, 400watts 24-36inches, 1000watts for anything deeper (usually only used in public aquariums). This would be for the section of the tank covered by the particular fixture, so if you have a 6ft (lenth) tank, you would need at least 3 of fixtures of whatever wattage you choose.
Also, temperature of the bulbs are important. The lower the rating down to 6500k will cause a lot of growth, but will cause other things to grow faster also such as green algae. On the higher end 22000k will have very little growth and will be almost purple. Many go with 10k or 14k for a good balance.
