Apache getting hammered... Wordpress site. CPU 100%

Hi folks. I hope one of your gurus can point me in the right direction.

I have two servers, one for Apache and one for MySQL, 4GB and 2GB RAM respectively. They run about 10 wordpress sites with very low traffic, and resources have not been a problem.

I now added another WP site with about 3000 pageviews per day and the server that runs Apache is sitting at 100% CPU all the time. Page load speed is dismal, somewhere around 6 seconds.

The site is www.fairtattoo.com. It has about 5000 posts currently.

Plugins are W3Total Cache, SEO by Yoast, WP Optimize. The theme is a professional theme from mythemeshop (truepixel) and I have used it without a problem on other sites.

Apache uses mpm_prefork.

Strangely, apache will use only about half the available RAM, approx 2GB of teh 4GB available. I’ve tried tweaking mpm_prefork with a few variations, but no luck so far.

Here’s the mpm_prefork:

<IfModule mpm_prefork_module>
    # Default 5
    StartServers        5

    # Default 5
    MinSpareServers     5

    # Default 10
    MaxSpareServers     10

    # default 256
    ServerLimit     256

    # default 256, less or same as Server Limit
    MaxRequestWorkers   256

    # deprecated: Called MaxRequestWorkers now
    #MaxClients     128

    # Can be a high number like 20000 but do not set to 0 (unlimited)
    MaxConnectionsPerChild  20000

    # No need to Keep alive, just keeps threads busy
    KeepAlive On

Where next? How to diagnose?

Thanks in advance