I do not think that 18 watt pl-l is worth the money or hassle. I use the 55 watt pl-l in a tent and it works great, but for your pc case I would just use some 26 watts cfl if you can find them. Remember to have a reflector worked out for the light, makes all the difference, like twice the light with mine.
You want the lights sideways to the plant, less light comes out the bottom of a cfl. You can also hang them vertical in between plants, even more efficient.