Overload Of Traffic

I am not sure if my dedicated server is being attacked or if it is legitimate traffic. I need help figuring out the difference and if it is an attack, how to prevent it, and if it is legitimate traffic, how to configure the server to handle the load.

My server information is below:

HardwareIntel Xeon 3220-Quad Core [2.4GHz
8GB DDR2
SATAII 500GB

SoftwareCentOS 5.3-32
Apache2
MySQL 5
PHP 5
When I do ps aux|grep httpd|wc -l I get the count of current connected clients of 259 which is always maxing out my MaxClients of 256. I had increased it to 512, and it maxed out, I had increased it to 1024 and it maxed out, and lastly I had setup to 2048 and it works, but slows the entire server down.