Indeed, the 8400 GS was a quatum leap from the old and useless integrated video chip Intel 915GV, with a humble PCI card I am able to play Devil May Cry 4 on mid-high at 800*600 and Quake IV at ultra on the same resolution with playable FPS.
Some games however are a bit picky, just like Beowulf that ran like crap even at lowest settings, also Unreal Tournament 3 was "playable" but with lowest settings it looked a bit ugly.
Maybe that 9400 GT that amazon offers can deliver a bit more of performance since it's DDR2 128, while the 8400 has only 64 bit of bandwith.
Again I can only offer my experience and some wild guesses, but choosing between discarding a potentially obsolete machine or injecting it a little more life with a moderate investment, well I'd get that 9400 GT and see what happens.
.C.