Low Frame Per Second

Hi,
I have AMD Phenom II x4 3.2 Ghz ( 4 cores )
HD4890 1 GB 256 Bit ( iCooler x4 )
2 GB 800 Mhz ram

But I have 30-40 FPS.
Why ?

I have got high FPS in Crysis 2 Multiplayer Demo's extreme (gamer) quality.Or in Assassin's Creed : Brotherhood.

-Sorry,my English isn't good-

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