Well optical or 3.5mm connections will be fine
The system uses 3 3.5mm jacks for each channel (rear/front/Centre+subwoofer) and they are removable.
Optical won't be affected by frequencies (makes a noise when you get a phone call or message for example won't happen on optical), not sure about setting optical up, 3.5mm and RCA are basically the same thing and both are easy to set up.
I would use Optical if your sound card has an optical (TOSLINK) connector, if it doesn't then use 3.5mm.
Not sure on S/PDIF.