There are plenty of hardy, low light plants that will do well with say 1.5 watts per gallon. Some on these forums have just that and no CO2 and their tanks look very good.
I currently have only about 1.5 wpg but my plants are growing like wildfire....but then I do have direct sunlight too. But with all that light and growth you would think I would need CO2 to keep up with the plant growth and their growing needs for it.....but so far they haven't complained. They're still very green, no browning, and growing like weeds.
I will eventually move up to 2 wpg and CO2 injection though. Research and read up on your many options.....and above all....have fun!
OH btw my current plants are amazon sword and cabomba...though the cabomba is very delicate and you have to treat it very gently or it will come apart trying to plant/propogate it. Other than that a very easy plant to keep.