Since the only reliable way to preserve any of the ranking power of the original URL is to use a 301 redirect, then that's what you should do when you move pages to a new folder/directory. But you will probably see the rankings for these pages dip at first, and then recover most - if not all - of their original rankings.
It may take more than one Toolbar PageRank update cycle for the full effects to be visible, but Google's internal PageRank data will be updating itself continually. So don't let the Toolbar PageRank of the new URLs discourage you or affect your decisions for at least six months after you start because it's not liable to be very accurate until then. Watch the rankings and traffic.