I don't have personal experience w/ this but have friends who have. Don't know if you've seen this, but for lights for growing succulents, to be successful, the lights have to be within 3-4" inches above the top of the plants. No so easy to do (nor real attractive looking).

There are several kinds of lights that can effectively be used for plant growth.
Metal Halide (MH), High Pressure Sodium (HPS) and High Intensity Discharge (HID) are the types most commonly used in commercial growing. These lights provide extremely bright light up to 120,000 lumens over a wide area of approximately 4x4 feet (about 7500 lumen/ft) and are a reasonable substitute for direct sunlight. However, these lights generate tremendous amounts of heat and generally run at 400-1000W.

Home growers looking for supplementary lighting have traditionally used fluorescent bulbs. There are 3 main types of fluorescent tubes named based on the diameter of the tube.

T12: These are the oldest and cheapest fluorescent tubes with a diameter of 1.5 inches. A single 4 foot long T12 running at 40W will generate about up to 1980-3300 lumens (depending on bulb specifications) over the 1x4 foot area underneath (max 825 lumen/ft). This type of setup is very effective and very affordable but T12 bulbs are being phased out in favour of much more efficient T8 and T5 tubes.

T8: 1 inch diameter. A single 4 foot T8 runs at 32W and can generate 2850-3400 lumens.

T5: 5/8 inch diameter. These are the newest, brightest and most efficient fluorescents avaiable, and therefore also the most expensive. T5s are split into 3 types, High output (HO) Normal Output, and High Efficiency (HE). For the purpose of growing plants, you will be looking for HO bulbs. 4 foot T5HOs run at 54W and generate 5000 lumens. A fixture of 4 T5s will generate 20000 lumens over 1x4 feet (5000 lumen/ft) when coupled with reflectors, which will be effective for growing most plants indoors.

CFL: Compact fluorescent lights are bright and can be used to grow plants when space is an issue.

As you know, another option is to use LED grow lights. LEDs tend to be very expensive and the benefits over HID, MH or Fluorescent lights tend to be overstated by the merchant. Also unlike the other bulbs, LEDs do not generate light over a wide spectrum, rather they generate narrow bands of red (650-660nm) and blue (460-465nm) that roughly mimic the peak absorbance of chlorophyll (430nm and 642nm). While this is not necessarily a bad thing, it does mean that your plants will be under purplish light for 12-16 hours a day and won't look very attractive.

I currently use 4 3-foot T8 bulbs (about 2900 lumen/foot) with a timer set to stay on for 16 hours a day. I'm also moving in September and will be looking to upgrade to a fixture of T5HOs, which is also what I'd recommend for you.
I've just given you a quick overview but there's much more information on the "Growing under lights" forum.

If you decide to get fluorescent fixtures, businesslights.com is a great supplier and offers full fixtures with bulbs included.

Rennet-thanks for the information! That's a great condensed version & answers a lot of questions. We've been thinking of growing a few things under growlights. This gives me a great place to start.
Tally HO!

Thanks for the info. The T8s and T5s sound like they might be a better option. Would you know, off hand, how much heat they tend to give off and how close I'd need to hang them over the plants to use them? Again, safety concerns.

Aaaand I guess I probably should go bug the growing under lights guys :)

Oh, I did have another question, though. If I'm growing my plants on a windowsill, would an open window allow significantly more light in than a closed window, or would it not make much difference if everything's within a few inches of the window already?

1. T8s and T5s run cool enough that you can place them as close to the plants as you'd like without burning them. As long as the plant isn't in contact with the bulb it won't be damaged.
The fixtures don't give off enough heat to pose a safety risk. I can lay my hand on the top of the fixture, or touch a bulb after it's been running for 12 hours with no discomfort. The bulbs may be very warm, but they won't burn anything.
Since the inverse square law applies for growing lights ( Double distance = Quarter light intensity) I would place your plants no further than 6-12 inches from the lights with the highest light requirement plants closest to the middle of the tube where the light is most intense. The closer the better. You don't have to move the lights or anything though, just put things under the pots to raise them closer to the light. I grow a number of tropicals including Jades, Hoyas, Adeniums, Sansevieria and Jatropha under lights. The Jades, Adeniums and Jatropha flourish when placed near the middle raised to within 2 inches of the light, while the lower light plants can be placed at either end of the tube at a distance of 6-12 inches.

2. The growing under lights forum is a good resource, but is very, very, very slow. I would use it more for the search function than I would for garnering replies.

3. An open window will provide more light than a closed window. This is especially true if the window has any sort of UV treatment or film applied. However, if you are going to be supplementing natural light with grow lights then the difference between an open and closed window is not a major concern.