You don't need a S/PDIF cord running from your videocard or have a seperate soundcard. The ATI HD 3870 has its own integrated 5.1 channel sound chip integrated into the videocard. They are confusing nVidia with ATI videocards. HDMI will output both video and audio on the ATI HD 3870.
Just use the HDMI adapter that came with the videocard. Hook the HDMI cable from the videocard to the TV. Then choose "ATI HDMI Output" in the sound playback devices in Windows.
I know this because I hooked up my ATI HD 3850 to my 65" Sharp LCD TV.
nice screen lol