High Trafic

Hello
I have a website installed on wordpress i am verry satisfied by wordpress but since my website trafic increased i have some trouble… i have a medium of 40.000-50.000 visitors/day and more than 200.000 views
The problem is that from a recent time… when i have a high trafic… my mysql create a high trafic… because of mysql my dedicated server CPU goes up to 900 usage… 1st i tought it was because the high trafic but it did the same thing when i had something like 200 visits in 10 mins.
What i can be the problem because this high CPU usage?
I have Supercache and db optimize and i dont know what to do more than that to avoid this high cpu usage.
Server caracteristics are:
* Procesor Intel Core 2 Duo E7400 2,8 GHz, 3 MB L2
* Front Side Bus 1066 MHz
* RAM 2 GB DDR2
* HDD 250 GB SATA II 32MB
* Trafic 6400 GB / Month
* Intel Gigabit Ethernet
Thank you and i will be grateful if someone can help me with some support.