A 'planted tank' can fall anywhere in quite a broad spectrum of expressions of the idea of 'planted' - anything from a betta bowl with some java moss to one of those gazillion gallon living art masterpieces that make us drool.
Speaking for myself, it's the process: the learning, the experimenting, the tinkering, the time on these forums, that make up about 70% of the 'worth' in having a planted tank. The tank itself is the other 30%.
For me (and my family) having these tiny, thriving (usually) ecosystems in our living space makes a huge difference in our outlook and our mood. I find myself more interested in watching the tanks than the TV. If something is out of whack, or not what I expected, I get to interact with a community of enthusiasts who are happy to help and it's great to be a help when I can chime in.
From the question, I'd guess you're a little intimidated, but there's no need to be: check your lfs, ebay or aqua bid for anacharis, java moss, jungle val - pretty easy plants that will do ok in your set up as it is, buy a small (250 ml) bottle of excel and dose your tank for a month - make notes and watch the plants and your fish. Even a little planting can change the fish's behavior. Get excited and check youtube and the boards for the plans to put together your own DIY CO2 - it won't support a mega planting in a 55 but you CAN see the difference and it's FUN.