Replace that 8400GS with at least a GTS 250, or 9800GT. And overclock that quad core from 2.4Ghz on up to 3.6Ghz if you can. If not, I know it'll at least do 3.2Ghz. Raise the RAM up a bit too, but not too awful far cause its probly not overclocking RAM. Then see if the game runs any smoother. My bet is that it will.