A relatively quick survey.

[eluser]Dam1an[/eluser]
What you could do, is use the existing users table for authentication, and then use your own table which has
user_id
alias
post_count
thread_count

That way you no longer need to do the join, and they can have a differant username on the forum if they want (very useful if the site does authentication by email... as you don't want your email on show)