Does my PC support dolby digital & will a soundcard be better than my onboard audio?

How do I tell if my PC supports dolby digital or DTS? I read that your PC must support this to send it to a receiver.

I have the ASUS P5K PLVM motherboard which apparently supports 6 channel HD audio. I do not see the outputs for this so I figure this must just be for computer speakers. I am looking at getting an audio card to send audio via digital optical to my receiver. Will this also be HD quality audio? I have realtek HD audio manager on my PC.

Edit - Found my answer. I need a S/PDIF output module to connect to my motherboard. I don't need a video card. I still don't understand the dolby digital and DTS. Does my computer support this if I have certain codecs or is there something else to it?