Need help with server high load

Our domain specification
Dual Xeon 2.4 HT
4GB DDR Memory
2 x 200GB IDE
WHM/CPanel

My server hosted about 470 domains and usually load over 20 and 40% Memory.
I do not understand why a dual xeon server can not handle just 470 sites?
Which command i need to run in SSH to provide useful information for you to help me?

Thanks in advcance.

 

 

 

 

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