I am doing something similar (http://examancer.com/) and I use a single table. If its new, it board id # is set to '0' (each board id # has a different setting) and the parent id (the id of the parent post, if its a reply) is set to zero. If its a reply, the parent id feild is set to the parent post's id #. So, basically, the values of the parent id feilds and the board id feilds give you quite a bit of info on the post, and along with the other standard feilds (post id #, subject, text, time posted, time edited) you have a pretty complete comments based system for news articles, plus a whole forum system all in one table.
