Ethan, there is a problem with hosting your site at a free phpBB host. They usually do not let you move it when you decide to upgrade to a professional solution. So my advice would match that of hmalekib: go ahead with a paid solution now. You'll be better off in the long run, especially if your forum is a success.
Daniel B., CEO - Bezoka.com and Ungigs.com Hosting Solutions Optimized for: WordPress • Joomla • OpenCart • Moodle Data Centers in: Chicago (US), London (UK), Sydney (AU), Sofia (BG), Pori (FI) Email Daniel directly: ceo [at] bezoka.com