According to wikipedia, the lumen (symbol: lm) is the SI unit of luminous flux, a measure of the perceived power of light...luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.
Plants work off different spectra than the human eye so the lumen isn't really a very useful unit of measurement. I would suggest you browse the cfl growing forum to see what other people with a grow space similar in size to your own are doing. The trick is to get as much light as possible in there without heating up the space too much.
One 18W bulb isn't very much light, even for seedlings. You might want to think about 23W or 42W bulbs, two to four for each plant. "Daylight White" (6500K) are good for vegtative growth. "Warm White" (2700K) are good for flowering.