Dim Tank

I scored a totally free (out at the curb with trash) 20 Gallon Long that I cleaned up, tested (no leaks), and it is totally ready to go. It has a working Penguin 150 Biowheel filter, black plastic hood and two Whisper 40 air pumps that work perfectly. It also came with a standard single tube lighting fixture, which is the only thing I have a complaint about (looking a gift horse in the mouth, I know). The thing is that the light seems very, very dim. It had a 24" 18W T8 in it, and I picked up a 20W T12 to see if it improved any and it's still awfully weak seeming. I'm probably comparing it too much to the 10 gallon with 2 20W screw-ins on the other side of the room, but I'm afraid I'll barely be able to see my fish under this light, much less grow the obligatory java fern that I already "planted" all over the peice of driftwood I put in the tank. Are 24" fixtures over 20L tanks notoriously dim, or should I see about finding another light? Anybody have any thoughts? I happen to have some 40W ballasts laying around. Would it be possible/worthwhile to overdrive the bulb? I feel like the fact that it is a 24" light over a 30" tank hurts it some, and then the plastic hood doesn't help much. Would a glass cover improve it any? Too many questions, I'll stop now.

Thanks for any feedback.
- Kevin

I'm trying to make this tank as cheap to get running as possible and the more original equipment I can use the better. I'll look into a new fixture if I have to, and that sounds like a good one, I just want to keep my expenditures down.