He made no mention of gaming, only picture quality
In other words will there be a significant improvement in picture quality if I upgrade the graphics card or will the difference in quality be negligible?
If you're playing games or watching HD video, that's a different story and the 8400GS won't be able to handle HD video at 1080p.
And the 8400GS does do HD. worst test I saw showed 43.7% CPU usage on a VC-1 DejaVu Movie
There's also apparently 2 versions of the 8400GS G98 and G86, according to this image