It will be tough finding a sound card with a S/PDIF out in header format. The only card I've seen with a S/PDIF header was an Audigy 2 ZS, and even then it was S/PDIF in rather than out.
Additionally from what I can tell based upon NVidias documentation, the 8400GS's S/PDIF connector is also input rather than output. The card itself provides audio through HDMI or DVI via a built in Realtek chip that is integrated onto the card itself. All you need to do is select it under sound devices to enable it.