I also have an NVIDIA GeForce 8400 gs : http://www.newegg.com/Product/Product.aspx?Item=N82E16814133364&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-RSSDailyDeals-_-na-_-na&AID=10521304&PID=4176827&SID=1errfiofrg8zo
To get the latest drivers for your video card, go to this link: http://www.geforce.com/Drivers
From there, you can press the button "auto-detect your gpu", and the program will scan your computer, taking you to the page where you can download the driver.
On my 8400gs, I'm currently using the 295.73 driver and it works wonderfully. Watch out for the 296.10 driver, I heard it isn't too good. Hope this helps.