This may be of some help from you as I just came down the same road...
I have a 30G planted aquarium. I started with one 24" Power Glo fluorescent. The two plants I had did ok, but all this is saying is that they didn't die. One month later I bought lots of planst with varying light requirements as well as purchased a new canopy ($100 CAD) that would hold 2x 24" fluoro's, so I used a power glo & and an aqua glo. My plants looked better, but not great. Another month later I discovered I could have both canopies on together for a final total of three 24" fluoro's (2x power glo, 1x aqua glo). All my plants looked decent after this. A couple weeks after this I added CO2 via a store-bought hang-on-the-back unit (yeast) with a bubble diffuser. Within a week all the plants looked great. Growth rate has increased noticably and the plants look much healthier. I have plants with high and low light demands and they all look better than ever, but still with plenty of room too improve.
The moral of the story, upgrade your lighting first. Once you've hit at least 2W per gallon you can start thinking of adding CO2. Right now, light is your limiting factor in good plant health and growth. See if you can fit both your light hoods on together as I did (with 1/16 of an inch to spare
Or if you are not afraid of using a couple tools, you could consider what I'm now doing.
I am now in the process of building a new canopy designed for up to eight 8 compact fluorescents for my 30G. I'm going to use 6-8 23W spiral compact fluorescents for 4W-6W per gallon. It's a simple and cheap DYI model. It's a wood box with an open bottom and a hinged top, built to the right dimensions to sit on the inner lip of the aquarium and still leave a couple inches of room for the heater and filter on the back. I am building it to the same dimensions of the glass canopy that my current lights sit on. Inside will be two cheap bathroom strip lights, each intended for 4x 60W standard light bulbs. I'm a bit worried about the heat generated from the compact fluoro's in the enclosed wooden box, so to counter this I'm drilling plenty of holes for airflow and will monitor with a thermometer. If the heat buildup is still a worry I will cut out a hole in the back small enough to attach a cheap computer case fan. The wood (oak laminate) is huge with enough left over to build at least 3 more. They're cutting it for me at the hardware place if I give them measurements. I just have to screw it togther and attach the lights. I live in an apartment where my only power tool is 12V drill. Anything I build is as KISS as can be.
Hope this helps!
Colin