For those with a deeper technical interest in Web server technology...
Since we moved to the new dedicated (VPS) server we have implemented a feature called memcache server and turned it on for vBulletin.
48 critical system items are stored in the cache which include things like the forum permissions table, system options/settings, all the plugin code and so forth.
These items are stored as serialised objects which means they need minimal processing when loaded from the cache.
What this means is that any time one of these items is needed it is retrieved from the cache in memory and not from disk or the MySQL database.
As many of these items are retrieved on every page hit, we have saved many disk/MySQL requests. In fact more than 18 / second or over 6 million in the last 3+ days.
This single change has helped performance more than anything we've been able to do in the past,
it means that pages are delivered to you faster than before, and more importantly we can handle a larger number of concurrent users.