I am new to the HTPC concept. But I have been using linux for a while now. I have a Sony HDTV with a DVI in, and a mother board which has a 5.1 analog out to my speakers.
If I get the pcHDTV tuner card and use MythTV, will I be able to get 5.1 sound from the HD channels directly from my mobo's 5.1 analog out? Or do I need to get a AV receiver?

Thanks in advance for all the help.

if your soundcard or the driver can decode an AC3 or DTS stream, you should be ok to receive 5.1 over your analog outs.
I use digital out the a 5.1 receiver, so I don't exactly know what driver or what soundcard you need, but the passthrough digital SPDIF out works with mythtv and 5.1 over OTA HD.