Which cable for SPDIF? Do I need a specific one?

You need a cable that is 75 ohm (such as a good video cable or RG6 antenna wire) with an RCA at the output end and probably a mini plug at the PC end. You can use most audio cables in a pinch.
If you have a cable with RCA connectors at both ends you can use an adapter at the PC end. If you want to use an antenna cable, one that has F connectors at both ends, then you can get adapters for both ends.
You can get cables specifically for digital audio from Monster Cable, Apature, and many others. These will mostly have RCA on both ends.

SPDIF - is a digital audio format, used for transporting digital audio signals by using coax, twisted pair or fibre optics cable with mini jack, RCA or Toslink.

You may need something like these for your setup.
Option 1 - http://www.tigerdirect.ca/applicat [...] &CatId=450 or http://www.projectorpeople.com/acc [...] 73064797-2
Option 2 - http://www.huetronworld.com/dataca [...] slink.html
Option 3 - http://www.huetronworld.com/rca-3-5mm-12ft.html - by using only one end of the RCA connector, usually the black or white (RCA) and just tape the red (RCA) end.