There actually is motherboards with better audio for HTPC.
These have better signal to noise ratio.
You don't really need a sound card, audio outputs are on the board.
and you can run it stereo, or up to 7.1 or whatever, it's switchable.
and it has a subwoofer out, etc...etc...
The board should not have on board video, you should use a video card.
The top of the line video card, I found had a better picture quality.
(the bigger the screen, the more critical it becomes).
You should be using a quad core processor for best video quality, and 4 GB of RAM...
at least. Dual core I would not consider for video quality.
So here is a system couple years ago built, specifically for HTPC as an example:
(you will see that these are older models)
Computer, complete parts
Quad core AM3 motherboard w/ surround sound outputs and 3 Ghz processor, high speed 4 GB memory, high speed hard drive (for programs), High speed HD dual output high resolution video card, Dual input HD tuner card w/ remote and record function, 550 Watt power supply, Tower case w/ fans, Bluray HD CD DVD burner, Mouse and keyboard, Windows XP Professional, Office Professional. (All hardware compatible XP to Windows 7 or future 64 bit operating system)
Phenom Quad core 965 Black Processor UPGRADED faster processor
LG Electronics 10X Blu Ray writer UPGRADED faster DVD writer
Radeon 5870 Graphics Card UPGRADED lifetime warrantee, better card
Happauge HD TV dual input tuner/ remote UPGRADED 2 switch able inputs w/ remot
Windows 7 Professional w/ media center UPGRADED watch / record HD TV, etc…
Better graphics resolution, 64 bit memory usage.
G. Skill 8 GB high speed memory UPGRADED doubled original memory
Antec earthwatts 600 power supply was a better choice due to energy payback.
· Will use 10,000 RPM drive for programs only
· Will use 1 TB drive for files only (video and office files)
· Will use all memory with 64 bit Operating system and 64 bit Processor
· Cost of upgrades was $350
You can use the motherboard's on-board sound for its optical connection. Do not use the motherboard's analogue jacks, they always sound horrible, but the digital connection will be fine, especially with the higher-end realtek chipsets. If you wanted to use analogue connectors, I would say go get a sound card.
You will get better advice for components for a HTPC here: http://www.avsforum.com/f/26/home-theater-computers
I don't think you need a video card unless you want to do gaming. They will add noise and heat. On the other hand, home theater gaming is awesome, I love playing Dirt 3 on my 125" projection system!