There is one simple way you can double the speed of your server, and that is to install IonCube's PHP Accelerator - it is fast, free, and works perfectly on Linux. If you don't mind paying money and want another 20% extra speed boost, Zend's Performance Suite is the way to go - it is better supported than PHPA, works with newer versions better, and has the edge on speed to boot. If you have a small/non-existent budget, PHPA is your best choice. If you have a little money and really do want to push your PHP code to the max, Zend's Performance Suite will be a purchase you will not regret.
Recently the Alternative PHP Cache (APC), the cache that performed most poorly in our tests, has been transferred into PEAR, the PHP code repository - it is now being worked on by PHP developers. As such, it is now much better than it previously was, almost snatching the #1 spot. Well worth a try!