Please help me optimize my Apache and MySQL?

Hello, my linode is Linode 540. Everytime I put one of my wordpress powered sites on it, which has a most user online of 240 and about 9K daily traffic, all things stopped within 5 minutes. The pages wouldn't load but a HTTP 500. Seemed apache shut down. I had to reboot my linode to login via SSH.

All errors were almost the same:

WordPress database error MySQL server has gone away for query... 

I submitted a ticket. They told me to "tune the Apache MaxClients directive to limit the number of connections". Honestly I do not know how to set that numbers.

In my apache2.conf, it's:

 <ifmodule mpm_prefork_module="">StartServers          16 
    MinSpareServers       8 
    MaxSpareServers      24 
    ServerLimit         400 
    MaxClients          200            
    MaxRequestsPerChild   500</ifmodule> 

I think it's using prefork mpm (dont know how to check that out).

My control panel is ISPConfig.

I have been too afraid to put that site on my linode. Several months ago I've tried several times then finally I gave up. Now I have to face it. The shared hosting wont allow the site run on it any more.

If any body can help me out, it would be so nice and I'm so happy.

Thank you so much!