Website performance tuning resources
Both of the above links contain a lot of useful information about how to improve the overall end user experience and ways to gain metrics on site performance from an end users point of view.
Other things that can be done to reduce the current load on dedicated servers to increase its capabilities are:
- upgrade third party applications to latest versions which may include speed improvements
- reduce memory usage of apache by removing unneeded modules.
consider using some form of php caching like http://eaccelerator.net/
- investigate using a small light weight web server to serve static content on a separate sub domain like images.yourdomain.com.au
- optimize your sql queries to reduce page loading times