a forum site take too much cpu of the server

high traffic forum
500 --600 people online everytime
and 1600 members online when the highest .
often take too much cpu and system resources
and the host compoany warning me
what should i do ?
i don't put any ads on my forum
so i don't have enough money to a decatied server.

and which forum software take less system resource?
compare vbulletin,phpbb,wbb ,invisionboard ,ubb?
do you have any experience or good suggestion?

 

 

 

 

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