I've done Search Engine Optimization professionally. PunBB is pretty friendly with Google.
For example, on this page, key words are covered in three main locations...
If you try a Google Search...
This page comes up as #1.
There are other factors that determine page rank. PunBB is Google friendly, but that alone may not get you top listing. If this is a huge concern to you, maybe you might want to check out Google's Webmaster tools...
Heh... or you could just pay an SEO consultant a crazy amount of money