Like right now my site www.rpgtutorials.com Is using a phpnuke with intergrated phpbb. Will google index every single page including threads+posts in forums?
Just use a sitemap generator, e.g. A1 Sitemap Generator, crawl your site and create a Google XML sitemap. You may want to use some crawler filters (perhaps even regular expressions - if so, PM me for link to newest beta) to filter out duplicate content pages.
It's not enough just creating your site and forum. You've to keep your content up-to-date. And also have some good backlinks. But beware of Sandbox At last, note that your site should be sef and seo compatible to get more visitors from search engines.
sitemap can be make for every types of cms. i suggest you to search for gsitecrawler . it's the best software that i know know for creating sitemap