Missy,
A standard 55 gallon tank is 48" long, and you say you have the lights that came with it. Are they two short bulbs, put in one line to stretch the length of the tank, or are they two full-length bulbs placed next to each other?
'Standard' flourescent bulbs seem to be rated at 10W/foot. So, if you have two half-length bulbs, you probably have 2x20W=40W of total lighting. If you have twin full-length strips, you would have 80W of lighting.
If you have 40W, that may be tough (or not, seems peoples' results vary). If it's 80W, you should have decent luck with low-light plants.
I can't comment on substrates, CO2, 'high-level' lighting, etc., because I also don't feel that I can justify the expense for them (right now

). Here's what I have going, for info:
Right now, I've got a 46 gallon tank, with 30W of (10000K, 'Aqua-Gro') light. Regular gravel as a substrate. I use liquid ferts occasionally (Flourish and Flourish Excel -- a four-to-six month supply cost me $18 online). The plants that do well there are Java fern, Java moss, crypt wendtii, hygrophilia, and Amazon sword. Also, surprisingly, I have some Cabomba (sp?) that's doing almost too well, even though everything I read says they need more light. I also have a few species of Anubias (barteri, nana, etc.) that I'd say is doing well enough -- some leaves yellow and die, but I get many more new leaves to offset them. A couple of moss balls that seem ok, but they never change, so who knows

. I also have some anarcharis (sp?) which grows well, but my gouramis and angels seem to like to eat it.
In short, personally I think you could, with a little research, get pleasant results without the expense of extra hardware. Of course, the results would almost certainly be better with the extra equipment, but that's a personal decision.