I webmaster 2 sites on dedicated servers (they're not mine, I just take care of them). We are switching to a web farm for performance (RackSpace). The time to change is when your site becomes unacceptably slower.. vBulletin is a resource hog. Our servers are constantly running at a maxed out load (that's why we are going to the server farm).
One option to consider before switching to a dedicated server (it can be expensive) is to start caching the pages. a good 'quick and dirty' cache script is at
http://www.ilovejackdaniels.com/php/...output-in-php/
Caching allows you to reduce your server load. Instead of creating a page EVERYTIME someone views it, you create the page every 10 seconds (you can set your own length of time).. So, pages that receive lots of views (50 per minute) only are recreated 5 times instead of 50..
Forums tend NOT to be cached.. You want to make sure the users are able to see their post after taking the time to write one. But, it is an option for webmasters short on cash.
Brett