Joseph wandered in from the void and babbled something like:

Currently on my tank I have the single strip aquarium light reflector
that came with the tank. I have a 40w bulb (common enough item), and
so I average about 1 watt / gal.

I hope it is flourescent. Incandescent bulbs are worse than useless.
If it is FL, You might get some very low light plants to grow under
this lighting. As a plus, algae will have less of a tendancy to
proliferate under thses conditions. Crypts come to mind...

What's the cheapest way to increase my lighting levels? Do they make
120 watt florescent bulbs? Or do I need to change the fixture itself?

If it is FL, the bulb and ballast are a matched set. You cannot just
change the bulb to a '120watt' bulb.

Can anybody point me to some good DIY sites on this? And while I'm at
it, how bright should I go so that I can grow most hight light level
plants?

Su
http://www.thekrib.com/Lights/index.html

You have given no information as to tank size/depth, desired plant
species, etc. - so specific recommendations are impossible.

Greg