I'll chime in with a bit of advice for those with busy sites. A while back I was working with the folks at thedieselstop.com. They'd routintely get several hundred users online and at times it would start to slow down a bit. We found that making a few changes to some settings made a huge difference.
The main ones we changed were the number of posts displayed on the postlist screen and the showflat screens. By default they had these set to around 50 or so. These settings apply to all unregistered users and are the defaults for new users. Many times they had around 200 unregistered users. So, that's roughly 50*200 posts to process when they visit the postlist screen and showflat (depending on how many replies are in the thread). Obviously they probably aren't all viewing these screens at the same time but the potential to process ALOT of posts is there.
So, they bumped these numbers down, I think to somewhere around 20 or so. This made a big difference in itself. This doesn't effect the users that are logged in as they get to use their own personal settings for the number of posts displayed.
So, just note that these 2 settings can make a big difference on very active forums.
