a few things...
yes the reason you are gettin 30-40 fps is your CPU.. the physics and such are all CPU dependant and sorry to say but yours just doesn't cut it.. If I honestly were you.. I would take back the card for a refund... with the cash, buy a new cpu.. or even upgrade to an AMD(will require new mobo and cpu) if your that concerned with gettin the extra FPS then buy a cheaper graphics card such as one of the x800xt's/6800GT.. whichever you prefer, not sure if you are totally an ATI person or not..
the x850xt is a high end card... your system is just kinda in the middle.. you should even it up a bit.