Try to fine-tune Apache (for example, decrease KeepALiveTimeout to something between 2 and 5 seconds): http://www.howtoforge.com/configurin...um_performance
Also, please use a PHP opcode cacher like Xcache or APC.
If you web app supports it, you can use memcached and/or tmpfs ( http://www.howtoforge.com/storing-fi...ory-with-tmpfs
) to reduce disk I/O.
You should also check if you can fine-tune/optimize MySQL (for example, reading InnoDB tables is much slower than MyISAM).