i had a very similar problem with one customer.. 2.6 gig DB.. over 2 million posts..
1. tuned my.cnf properly -- mysqltuner.pl is very helpful
2. split the forum into 2 (archive.tld.com and forum.tld.com)
that made the forum.tld.com -- which is 95+% of the traffic scream.. zoom zoom..
then the archive.tld.com was used for searches back in time etc etc..
3. modified the DB for ubbt_POSTS to have a natural reverse time based index.. this made searches 10x faster..
4. added an 'archive these posts' option to copy from main to archive, when needed
works fine
so i wouldn't call it 'simple', but it works very well
