I'd suggest something similar to this. It's what I use, and it hasn't failed me yet.
Have one for topics, posts, and users.
Topics: You'll need a topic ID, a title, and for sorting & entire topic deletion purposes I use a first post ID and a last post ID. Simply...I sort the topics by last post ID, and in my deletion page I check if the post about to be deleted is the first post, then that would delete the topic AND any posts with that topic ID. Also, user ID can be used here...or you can just grab it from the posts.
Posts: You'll need post ID, user ID, topic ID, and of course the body.
Users: You'll need user ID, username, and other info. Like passwords and shit.
That's a pretty straightforward way of setting it up, and it's pretty foolproof and secure. The way you put up is a bit crazy, but similar in some ways. With this way, it's easy to set up stuff like number of replies, organization, deletion, and whatnot.