It would help if you could give us the size of the tank. width, length, depth.
I can't just say get a 175watt Metal Halide if your tank turns out to be 30 inches deep. So I can give you a little basic knowledge.
A MH normally covers a 2x2 foot area. The wattage of the light depends on how deep the light needs to penatrate to reach the bottom of the tank.
150 watt 14-16inches
175 watt 16-18inches
250 watt 18-24inches
400 watt 24-30inches.
Example: If your tank is 24 inches deep.
If you want to keep a nice mix of corals mainly softies, anemonies, with maybe a few SPS (hard corals up high in the tank) Then you wouldn't need to have the max lighting, then you could get away with using 250watts of light.
But if you plan on a large varity of SPS's, clams, anemonies, and have them even at the bottom area of the tank, then you should consider using the maximum lighting of 400 watts.
So if your tank size is 48inches long, and 24 inches deep then you would use either 2x250, or 2x400. This is for a reef tank.