the 8400 gs itself is quite old. if the driver doesn't match your os (32 bit driver on cd vs 64 bit os) then download fron the nvidia site. imo if the driver doesn't break anything, no need to upgrade it. if you're facing problems, download the new drivers from nvidia's site.
The Nvidia drivers at the Nvidia site are the latest, and they have updated drivers for legacy cards too. If you want the latest version for your card you can download it there. But, as always, backup your machine before you do any major change (and video driver IS a major change).