...Given that it creates an item for every post in every thread, 20 seems to be a good place to start.
Remember, you can also use the 'sa=news' function to get links to only the updated "topics."
As for RSS feed links, once we do the update to the forum and consolidate our themes, there will be RSS icons for each board, feed buttons on the main index, and automatic identification to the browsers that support feed discovery.