Re: Pure video = VDPAU ?
Well today I had the opportunity to plug the 8400GS card into a different machine - a Dell Dimension XPS Gen4, 3.6GHz P4, running Windows XP SP3.
I loaded the machine with the latest nVidia drivers for Windows XP, and set the BIOS to make the PCI slot the primary adaptor (the default was the existing 6800 PCIE).
On power-up, I saw the 8400GS announce itself in text, then the Dell BIOS splash screen, and then the splash screen for Windows XP appeared. All good, it got a bit further. After that, the screen went blank and the system froze.
I am sending the 8400GS card back for a replacement - clearly this card is faulty, or it only works with the PNY drivers - which isn't an option for me.
I'll post what happens when the replacement eventually arrives.