I'm preparing, at least psycologically, to HDTV on my PC. My current CPU isn't so great, but I have an external amplifier 120w x 6 that I don't what to throw in the trash soon.
When the 1920x1200 1080p monitors, and the rest, will decrease the price I'll be able to do the big step without any bad surprises (I hope), until now I try to taste how my last generation home cinema could be.

Some technical considerations

Dvd audio is AC3-5.1 and external digital receiver plays it with no problem via SPDIF.
Modern videoes 1080p, 720p, MP4, mov, Matroska (mkv) often use audio compressed in AAC-5.1, this is good because AAC is more efficient than AC3, but that is bad for my receiver that, as commonly for a receiver, it supports only the AC3 format.

So, how to use 5.1 with new formats?

You have to buy an AAC capable hardware receiver or convert AAC to AC3 at runtime.
This is theorically doable on Linux, and practically doable on Windows right now, thanks to a wonderful tool called AC3Filter.

Configuration

If you hears some strange beeps probably you should try to enable/disable PCM.
I advice to keep AC3Filter everytime on system tray to see if the encoding works well.

If everything works the output should be something like: