How should I tune apache when I see high CPU, but low memory usage?

It is probably, that some PHP scripts running in mod_php consume too much CPU time. Overall recomendations are general and each of them will have some benefit:

• Install APC cache and check that is really running (APC has bundled apc.php file which shows APC status and cache hits and misses)
• Install nginx and install it as a reverse proxy in front of apache - it will reduce overhead of serving static files and leave much more CPU resources to server dynamic content
• Add CustomLog to apache with %t %T directives and examine the log - you will see wich requests are running the most time, thus consuming most CPU
• optimize/rewrite those scripts