http://www.guru3d.com/index.php?page=rivatuner
Download rivatuner (its worth learning how to use it first, even though on the surface it is very straight forward). As with all overclocking, proceed very carefully, turning up your clocks a small amount at a time. With each adjustment you make, monitor the cards temperature (Win7 does this using performance monitor --Ctrl + Shift + Esc--) and test your cards performance, best way in my opinion, is to run a game and monitor your frame rate.
I cant imagine you'll get much out of the 8400gs, but its all good fun!