Theres a little more you can do to decrease server load:
-- Zend Optimizer: Its free and makes some minor tweaks in how the PHP code is executed to improve efficiency by maybe 3-5%.
-- Alternative PHP Cache (APC): An op-code caching system slated to be included in the PHP6 standard release, it caches compiled PHP scripts and can yield a 10x to 25x increase in performance for a major reduction in CPU load. Note: These are also called PHP accelerators, others include e-accelerator (foss), xcache (free), Zend Platform ($$$$), there are others but most/all are no longer maintained so be careful of installing anything obsolete.
-- Memcache: Caches PHP values in memory but the script must be coded with support for it... Packages like Joomla, SMF, and I think MediaWiki include support for it.
-- Custom Build PHP: Check the DH wiki for an article on custom building PHP, you can further increase performance/efficiency by disabling modules you don't need.
-- htaccess: Audit your htaccess files, if you can live without htaccess then you can further increase efficiency. For deny directives move the files out of public_html and edit the scripts to include them via absolute path. For auth directives (username/password logins) use PHP and MySQL auth instead. Theres no alternative for mod rewrite (SEF "pretty" url's) but those are slow anyway. Note: Its essentially an automatic on/off flag to check for htaccess files or not on an entire virtual host, so if you have 1 htaccess file then you may as well have a dozen of them.
There are plenty of other tweaks and tips to increase performance but doing all of that should help your site play well with others in a shared environment.
Hope that helps,