Hi confused one,
S/PDIF is an digital audio transport format from source to destination, inventors are Sony and Phillips. This digital audio format can be transported using two kinds of medium, typical 75 ohm coaxial cable, which carries the same analogue audio/video signals as component or composite video and stereo audio channels or transmitted by light through an optical cable which usually uses TOSLINK connectors at both ends, whichever is supported by your source and destination. They are different cables with different connectors.
I don't know much about SACD, I thought SACD is probably just high sampling rate but doesn't offer multi-channel audio? I could be wrong on this, do a google look on SACD. I usually associate 7.1 audio channels with movies, where multi-channel audio is commonly used.
It's fine to use DVI to HDMI converter from your graphics card for the video only source, and using digital audio (either optical or coaxial) from your soundcard to your A/V receiver or amplifier. Looks cumbersome but it works just fine.
Looks like it does support multi-channel audio, but up to 5.1 channels with compression.http://en.wikipedia.org/wiki/Super_Audio_CD