Lighting will be one of your most important decisions in setting up your grow room. First, look at your room. Metal halide or High Pressure Sodium lamps emit a considerable amount of heat. Do you have enough ceiling height to raise your lamps 18 inches above your plants AT HARVEST TIME? The use of remote ballasts and ventilated hoods with travelers will increase headroom and allow closer placement of lamps to plants.
If you have very little headroom consider a TekLight T-5 fluorescent lamp. It is only about 2 inches thick, emits very little heat, and produces more lumens than a 400 watt metal halide. This lamp can be placed as low as a few inches from the plant canopy. This is a real advancement in fluorescent lighting.
How much light do you need? A 1000W metal halide will provide plenty of light for a 10' x 10' room. You will get better light "coverage" by using a light traveler or setting two lamps a few feet apart and using timers to alternate the lamps during the light cycle.
During the light cycle (the period the lights are on) you want the best coverage you can get. If you have one lamp centered over your crop the outside of the perimeter will not receive the same intensity as those plants directly under the lamp. You can compensate for that by putting your light on a "traveler" which will physically move the lamp over the plants or you can improve your "coverage" by adding lamps. If you add one lamp you have doubled your energy consumption.
Be sure to use timers which are rated for the amperage of your lamp and designed for inductive uses. Typically this type of timer will be called an appliance or equipment timer.
Metal halide lamps with "warm" bulbs have pleased many a grower from seedling through the blooming phase. If finances aren't a limiting factor mix metal halide and high pressure sodium. For a small to medium size setup a 400w lamp will suffice.