My first "real" attempt at a planted tank was a 5.5-gallon All-Glass Aquarium. I bought a 16" fluorescent strip light (All-Glass) which came fitted with some awesome 15" bulb whose brand I never managed to check. I was able to grow virtually *any* plant with the 14 watts. As far as lighting goes, I'm a strong believer that "more" is NOT necessarily "better."
A few months ago, I was having problems growing *anything* in my 2.5-gallon tank with the 12" 8W fluorescent strip light, but that was not the case when I first bought the light. Anyway, thinking I needed *more* light (but not wanting TOO much), I got an incandescent strip light so I could use a 10-watt screw-in PC bulb. Big mistake. The lighting didn't get distributed at all so that one side of the tank was way brighter than the other side. I started getting algae problems, my crypts melted from shock, and it was just a mess. So I asked around for some advice, and I ended up going back to my 8W fluorescent, replacing the old bulb with a Power-Glo, and I saw my tank do a 180. Actually, the crypts continued to do strange things for a few weeks afterwards, but I saw an immediate change in how my Hottonia palustris, Bacopa monnieri, and R. indica were growing. It was nothing short of amazing.
The other thing I couldn't stand about that 10W PC bulb was that it made everything look yellow and washed-out, even though it was 6500K. The spectrum wasn't very good at all.
So basically, I'm agreeing with Daemonfly, and I'd even go as far as to say that a _
single_ 14W bulb (if chosen correctly) would be good enough to grow *almost* any plant. It might not be enough for a Riccia lawn, but I wouldn't put it past a good bulb to grow beautiful chain sword or glosso (provided you've got good substrate, CO2 and fertilization).
You can get a 16" normal-output fluorescent strip light for under $25 from Drs. Fosters and Smith (of course that's not including shipping) and it includes a really good 14W bulb. Coralife makes a 20W screw-in PC bulb, but this will probably cost you almost as much as the regular fluorescent strip light.
Sorry for the long post. If you want really fast growth, go with the higher intensity and make sure you keep things balanced at all times. But for good, steady growth, the 14W will do just fine.
For good plants for a 5.5-gallon tank, I could write a whole 'nother book
! You should decide on what level of light you're going with, first.