Back-End-Optimization - Best practice

[BITS]

Hi,

I recently did a lot of front-end-optimization and followed a lot of the best practice guides which are available (Yahoo (Steve Souders), Google) on the internet or in books.

Independent from this - where I still have to optimize things - I am already thinking about the Back-End-Optimization. At the moment I am running my site on a VPS with CentOS 6.2, Apache 2.2 and PHP 5.3.

What should be the next step to optimize the back-end? Is there any best practice guide available with an order like Souders did in his book "High Performance Websites"? Should be the first step to switch from Apache to nginx?