I used to actually store the number of replies in a few ("replies") in the post table. If the post was a reply, this field was blank. Then I realized that was a waste of space and now I just do something like:

Code:
$replies = mysql_num_rows(mysql_query("SELECT * FROM posts WHERE parentID='$ourthreadsID'"));
I'm not sure what you mean by "create 2 instances of the same table?"

I'll be making a beta release of my boards as soon as I get all the admin/mod options built in (and I have to get the search woked out too) so you can check out my code (even though it's not treeview style like you want)