Have you considered to install munin and monit on the server to monitor and visualize the server usage?
Here is a short guide from nginx wiki about running a cgi script with nginx:
But I did not try that, so I dont know if it works. It might also be that apache is not the problem here and the ressources are taken up by the cgi script itself.
In any case, if you want to try nginx I would recommend to do this in a virtual machine first as you cant easily switch your apache setup to nginx without reinstalling the server. You can download a perfect server virtual machine which works with virtualbox here from howtoforge.