My second computer is located in my HT equipment rack which I use mostly as a jukebox. It's networked to my main computer and has access to cable internet. I don't use it for video so it only has a 64MB video card connected to the receiver via Svideo.
For audio I have a SPDIF adapter(AUD$30 plugged straight onto the motherboard and sits in an empty slot on the back of the case) which has Coax/Optical output connected to the receiver. I then set the PC audio to output through the SPDIF and thats it, all audio from the PC is sent out digitally which is decoded by the receiver(even DD and DTS).
This was simple, cheap and suited my needs.
cheers