I always suggest (for a cheap effective setup) to buy a versi-top and a double or triple strip light. That way you get effective watts per gallon for a modest investment, and it allows for upgrades should you decide later on to get compact fluorescents or T-5s.
Try some java ferns and crypts. I have those growing just fine under only 1.5 watts per gallon. I'm sure with 2-4 watts per gallon they would grow a lot more, but I'm okay for now.