I'm not a lighting guru, but I'll see if I can help.
There's a new types of lighting available in the hobby, I'll try to outline them here:
1. Normal output florescent (aka, NO)
There's are your everyday long tube florescent lights that many people have in their houses (or shops). You need a ballast to fire/light a florescent blub. A ballast for a NO fixture is typically small. NO lighting has very little place in the reefkeeping hobby. If you have a small fish-only setup, they can be used to light the tank and that's fine. Once you decide to keep coral, you'd have to step up your lighting (there's a few exceptions to this rule such as low light mushrooms and non-photosynthic coral).
2. Very high output florescent (VHO)
VHO lighting it still used in the hobby today, but starting to fall out of favor. A VHO fixture is much like a normal output florescent fixture, but uses a higher power and larger ballast to give the bulbs more output. VHO bulbs typically come in a daylight variety and super actinic. The daylight variety gives off a normal white light whereas the super actinic gives off a blue color. Many, many lighting setups are supplemented with a few VHO super actinic blubs to give the primary lighting a slightly more blue color. VHO lighting can be used to keep most coral. Keeping SPS (very high light demanding corals) under VHO is possible, but certainly not the best. Soft coral and LPS can thrive under VHO lighting. VHO bulbs do get hot, so some air circulation is good - but not hot enough to typically warrent a chiller. Common output per bulb is 110 (for 4') and 160 (for 6'). If you're running VHO only lighting, you typically combine a few daylight and a few actinic bulbs. If you're running another form of primary lighting (like MH), you just add 2-4 bulbs for supplementing the color. I believe the typical ballast can fire 4 bulbs.
3. Power compact florescent (PC)
You know those new style bulbs with the spiral bulb that many people are starting to use in their homes? That's a power compact bulb. They're basically small florescent bulbs that twist so that they take up less space. Based on what you mention above, it sounds like you have PCs. Given enough output, I would put PCs in the same category as VHOs. They can keep most coral, but some will not thrive. I run a 30" PC setup on my frag tank and I keep soft coral and LPS in their. Both thrive with that lighting. It has two 65w bulbs.
4. Metal Halide (MH)
Metal Halide is (for the most part) the standard choice for reefkeepers. Metal Halide systems get confusing. Bulbs choices come in two basic formats - single ended (SE) bulbs (like a normal socket) and double ended bulbs (often abbreviated as DE (for double ended) or HQI). Most people use SE bulbs, however I believe the output of DE bulbs are more, but they also cost more. MH bulbs also come in various colors (how yellow or blue they look to your eye). This is measured in Kelvin. You'll see something like 10k, which means 10,000 Kelvin. 6.5k is very yellow (the color of natural sunlight), 10k is mostly a white light, 14k introduces some blue and 20k is very blue. The color will vary depending on the blub and ballast you use (oh yeah, typically, each MH bulb requires it's own ballast). MH bulbs also come in a varity of wattage. Typically, the lowest you'll see used is 150 watts. From there, you have 175w, 250w, 400w and 1000w. Each produces more light and burns hotter. So, for example, a 400w light will produce a ton of light, but also a ton of heat. Because of the heat output, many people need to run a chiller when running MH lighting. I personally run a setup with three 175 watt 10k bulbs and do not require a chiller. The wattage bulbs you use typically depends on what you want to keep and how high your tank is (because water reduces that light output - so, less light makes it to the bottom of the tank). If I had to generalize, many that keep SPS use 250 or 400 watt bulbs. Many also say that tanks deeper than 30" should use 250 watts or more (but again, it depends on what you keep). To make this even harder, MH ballasts come in either magnetic or electronic forms. Magnetic ballasts are heavy, get hot are less efficient, but cost less. Electronic ballasts are the opposite and cost more. MH lighting also gives you the natural 'shimmer' look of sunlight hitting your aquarium, unlike other forms of lighting.
5. T5 florescent (T5)
T5's are quickly becoming popular with reefkeepers. They are much like VHO lighting, but the bulbs are much smaller, which allows you to use more of them. Each bulb also lasts much longer than your typical MH bulb (which most say should be replaced every 6-12 month, whereas T5 I believe is 12-24 months). With enough T5 bulbs, anything can be kept, including SPS).
6. LED
LED lighting is very new - the general idea is you have ton's of LED's that use little energy and give off a good amount of light. Each LED lasts years. This type of figure is also very expensive right now.
I'm sure if somebody else reads this they'll find something wrong, but that should give you the general idea behind lighting. So, the first variables you need to set yourself on is what size tank and what you want to keep. The choice should be easy from there.
For a 120 (assuming it's a 4' tank), a two bulb MH setup (with two 4' VHOs) would be great. Likewise, a T5 setup would likely suit you well too. Both would allow you to keep almost anything.
Best of luck,
Ron
A little pricy though.
