You may have more then 1 SPDIF connector on your card or it may be set up for more then a 2 pin connector, best thing to do is check your soundcards instruction book, it should be in there. I use onboard sound for mine and on the 790i the SPDIF out is literally right beside the first video card slot.It is awesome that if you need a small part, one that is hard to find or that will cost you more in shipping than it's worth, that a member on HWC will help you out.
In addition, even the 3800 series also has the DVI > HDMI dongle, which is where there HDMI support really started. I have been using a Sapphire HD3870 with one connecting to a 52" LCD and HDMI surround system and it's fabulous how the high quality video and audio are output so well, with one cable and mininal CCC configuration.
Lastly, I have been trying to figure out how to use the Nvidia jobbie. I have GTX 260 Core 216s running in SLi, have the cable and an X-Fi. I know where the cable goes into the GTX, and the sound card has an area labelled SPDIF. There are a bunch of pins though and only 2 on the cable. Which two do you use? I have been trying to find a manual which maps the pins.
It is an Creative X-Fi Xtreme Music for the record...