In other words, sitemap also lets webmaster to inform search engines that which pages to visit often and which pages to ignore, eg. visit to a single topic daily is not necessary however, particular modes need to be indexed every few hours as it get lots of new topics.
In WP, there is sitemap plugin which allows same.. so as Antes says, should have option to set priority, should be easy, its like appending custom values in xml of sitemap.
I would like to suggest that every board should have its own sitemap,so that sitemap can be divided easily.