there are some great free message board systems out there that will handle high volumes. phpBB is what I started with in the PHP realm of boards. There are many many PHP based systems that can be very successfull. There are also ones you pay for and sometimes that is the better route if you need consistant support. Within the last couple years I have switched over one of my big forums over to Invision Power Board and its just plain awesome and gets better with each release (www.invisionpower.com). You really just have to consider your needs and try out the different software to see what your most comfortable with. I learned alot more than I ever thought I would when I started my own webhosting, PHP and mySQL where only things I have heard of in the beggining and now I think they are the most powerfull backend components I work with today.

Most people see a forum here but there is so much more behind it, with a database and a combination of php and other languages driving it all.