Hmm. I would recommend you to use mkportal if you are using a forum script, such as ipb or phpbb, v bulletin, etc. Also if you are using phpbb then you can try phpnuke, post nuke, dragonfly cms.
I would recommend Joomla.
Set up a trial site and have a go at it.
Some webmasters have spoken highly of drupal for it's SEO benefits. I've not used it and therefore unable to comment. But I have used Mambo/Joomla and I'm very pleased with it.
Don't use mambo, its user interface and Admin CP are awful and so confusing. PHP Nuke or E107 are my reccomendations... check out http://www.opensourcecms.com for some demo's of CMS'
I'm a drupal user and yes, it does have some nice search engine friendly features built-in. I like the url aliasing, google site map generator and the ability to bring in RSS feeds without javascript.
I haven't used joomla though so I can't say one is better than the other...