Will adding an amp make a noticable difference?

I have a Harman Kardon AVR335 7.1 receiver which is rated at 55wpc.I am building a 7.1 system that currently consists of Paradigm 7SE MK3 for fronts and Paradigm CC300 for the center. As I find them, I will be adding side and back surrounds. This system will be used 99% of the time for multi-channel music. I listen to a wide variety of music and if by myself I like it fairly loud. Since most multi-channel music at this time is 5.1, should I be looking at 5 channel amps now and add another when 6.1 or 7.1 software becomes available? And I guess maybe the first question should be, is adding an amp going to be that noticable? If yes...how many wpc of an amp should I be looking for and what brands? More than likely I would be interested in used. Thanks for your input.