On my Dimension 8400 I upgraded the Video Card to ATI HD2400.  I did a clean install by deleting the other driver (ATI X300) first.  Installation went without a hitch, the card works great.  I notice in the Device Manager an unrecognized PCI Device.  If I uninstall it, upon next boot it will look for a driver, not find any and the question mark comes back. 

Looking at other threads, this card has no sound output and the Dim 8400 is using an integrated sound card. 

Please help, John