I would buy vbulletin, it's worth it. If you really don't want to pay anything though, I'd go with phpbb and put some seo mods in.
Some people rave about a lesser know open source forum called 'Quicksilver', it might be easier on the server load than phpBB. I'm going to test it someday. I don't know about the SEO part though, this info would probably be at their website or forums. Note: About phpBB and server load. phpBB has a 'template cache.php' option, which is very easy to install, and supposedly can reduce server load by up to 70%. See their installation docs for details.
I would recommend taking a look at miniBB - it does everything that you need. It is a bit more difficult to configure, but once set up it is super fast and really really easy to skin and integrate into any site. It is nowhere near as resource intensive as vB or PHPBB - yes, it is a bit bare-bones in terms of functionality, but there are around 20-30 addons (some free some paid) so you can setup your forums exactly how you want them. Unlike phpBB, you can even upgrade the core without having to re-install every mod you have ever made to it. 3 strongest features are: Best templating system of Any forum software I have tried Non-destructive upgrade of customisations SEO friendly URLs Don't take my word for it, install it and see for yourself. (find it at minibb.net)
Use MyBB with MyBB SEO Plugin The best combination! Although waiting till 1.2 might be better or you will need to re-design any custom skins you make.
Did you buy the $85 leased license? If you bought the owned license, it might be the support that expires, but not the license.
This DOES greatly reduce the server load, there is an amazing difference, but as far as the 70%, thats to be determined by someone who can figure that out ) But yes, phpbb is good