The game performance!

Hello i have buyed a new pc for the Dragon Age: Origins.

The pc is like this.

Intel i5 750 @3Ghz
3GB DDR3
500GB hard disk
GTX260 super OC
Gigabyte P55-US3L
Windows xp SP3

And i still have poor fps in the game. First i start the game and after i played a 20 min gameplay the game is running very slow. I have the 1.02a patch.

What is the problem? The game is still not optimized? Or is for AMD systems? Please tell me.