Of the two you list, the 4350 is the better value for watching movies--its cheaper and more power efficient than the 9400GT.
However, An 8400GS should be just fine for letting you watch HD video. The 4350 is a little better (and more power efficient) but probably not enough so to make purchasing the new card worthwhile. There is no reason to get another card unless you decide to start gaming/graphic editing. (or have a really honking big monitor)
A 300w dell PSU should handle the 8400GS or an 4350 just fine. Dell generally puts decent quality PSU's in their computers, and the 8400GS only requires 300w.
This graphic card hierarchy chart on Toms can help you compare different cards:
http://www.tomshardware.com/reviews/geforce-310-5970,2491-7.html
As you will see, both cards are relatively low end performers--but that's all you need for watching video.