Which motherboard for surround sound?

I am building a friend a computer. One of the things he wants to do is to be able to connect it to his amplifier and TV and watch movies in surround sound. What do I need to be able to do this? Does the motherboard have to have an optical or coaxial input or can I use the analog input? What DVD viewing software do I need to decode Dolby digital or DTS? I am going to be using an ATI 4850 so I think the TV viewing part is covered. I also am going to use XP 32bit.

Any motherboard with optical or coaxial output will have the capability to pass through AC3 audio (DD and DTS) and Windows 7 Home premium is all you need to play DVD's through either Windows Media Player or Windows Media Center.

If you are playing video files in a container format like mkv using different codecs like FLAC, ogg, AAC surround, then you will need to explore software (ffdshow) to pass surround sound to the receiver.

Blu-Ray, on the other hand, will require a 3rd party player to play from BD discs. I recommend Arcsoft TotalMedia Theatre 3 Platinum. Another option is PowerDVD.

What movie format does your friend intend to play on this system?