The current labels seem okay to me; they probably only have meaning to occasional visitors anyway, since those who haunt this forum regularly get to know the knowlegable posters.
I suppose there's no way to know it, but there should be a title like "posted hundreds of times but most of them are wrong". Seriously I kinda don't like the implication that a lot of posts makes someone a professional or guru. I gave up on usenet because of the trolls who posted dozens of times a day.
Here's a non-linear suggestion, you might try a feedback system like ebay. Instead of number of posts, have a "Rate This Post".