Right now I have this speaker system connected to the digital coaxial port of my ASUS A7N8X Deluxe motherboard. I have a second PC with a MSI KT2 Combo-L motherboard and SoundBlaster Audigy sound card ...

All you need is a cheap 1/8" minijack to mono or stereo RCA cable. However, using analog cables may be more optimal because you cannot get discrete surround gaming audio or DVD-A over the digital ...