One or more servers could help spread the load of a busy forum. If your forum runs pretty slow you could consider moving the web server to another one.

Server 1 -Web server, that hosts the program vBulletin
Server 2 -Database server, the hosts the mysql database that vBulletin uses

Another tip is to use datastore cache on the disk, and store attachments in the file system. Keeping the mysql database size down. You could also consider a slave server to use a database on the first server, or even a third server for the read actions.

Your final solution is a proper array of web servers and database servers and spread out the load, or use a hardware load balancer. But these solutions are quite expansive.

Another advantage of clustering with more than one server is you can save bandwidth. If you only have two servers (web + db), you can plug the db into the web server, and bandwidth between the db and web server will be free. If you have more servers in some sort of hardware load balanced cluster you can also save bandwidth behind a private switch. :)


Alex