No, an 8400 GS is basically for running two monitors together, not for gaming.
The first number in the nVidia model line tells you the series. The 8xxx series is about 3-4 years old now. Then there was the 9xxx series, then the 2xx, and now the 4xx series.
The second number indicates how well it performs. x4xx is low-end. You'd be much better off with at least x6xx.
I would recommend a 9600 card or better (i.e. 9600, 9800, 260 or higher, or 460 or higher).
This, for example: http://cgi.ebay.co.uk/Inno-3D-1024MB-DDR2-GeForce-9800GT-DVI-VGA-HDMI-/280552698765?pt=UK_Computing_Computer_Components_Graphics_Video_TV_Cards_TW&hash=item41523e738d
I might cost twice as much as the 8400GS but it's more than twice as good and will last you a while, esp if you do most of your gaming on the PS3. As far as gaming goes, the 8400 is no better than the onboard chip you already have (I've borrowed one before and it could only play TDU on the lowest settings).