Well, after doing some research, I found why the 8400 GS does not perform as well as the 8600 GTS.
I really needed my 9400 GT to do VDPAU. I hope Sparkle does not give me a hard time with it, but more importantly, I hope they just don't send me another 9400 GT if all their 9400 GT does not support VDPAU. Are there any suggestions on how to socialize this conversation with them. Would they even know what I mean if I mention VDPAU?