ISBoxer.com has a guide dedicated to this kind of question: http://isboxer.com/wiki/HOWTO:Tweak_your_framerate

To know what, if anything, you should probably be focusing on, see the "Identifying framerate bottlenecks" section. The tool in this section of the guide will help you identify whether it is your CPU, Video Card, or RAM that is the current limiting factor. You can also lower certain settings in the game for added performance at the cost of visual quality.

Do this first, then with a better understanding of the problem you can determine if you should be updating that 680 to a 980