Note that he is only interested in HD movies, not games. I too would like the answer to that question - would a 8400gs be enough, given that one doesn't care for games at all?
I would never suggest HD4350 as a gaming card. Its strong point is how well it handles HD playback. Ati cards have had the edge over nvidia cards in that field for the last generations. The HD4350 is a generation newer than the 8400gs but like I said it is in the eye of the beholder what is acceptable to some is not for somebody else.