poor performance on good PC

i have realy poor performance, bugy, like its a 15fps game on my PC which is pretty good
specs.
AMD Athlon II X4 640 3.0 GHz
4GM of ram
NVIDIA GeForce 8600 GT video card

so I think that is more than enough for this game so what could be the issue

You call the Geforce 8600 GT as a Good Graphics Card?
Hmm let me think about it and i think your Graphic Card is at Rank 97.

And of course it depends highly on the graphic options you have selected!