My objective is the ability to connect to my DAC or A/V Preamp via analog RCA or XLR or even optical from the fanless pc system. Which one is preferred RCA,XLR, or Optical? ? How can connection be achieve? ? My thinking sees a sound card with RCA outs ? ? ? But that my limited thinking. Video is not a major concern . . . . Music quality is.
Analog RCA would be the least preferred of those three choices. You can get good sound from analog RCA outs (don't mistake me), but it is the most susceptible of the three to hum and line noise problems, and if I were choosing between those three interconnections it would be my last choice.
If your DAC or pre-amp will accept an XLR input, XLR is a good option because balanced connections are insensitive to ground hum, and reduce other kinds of line noise as well. Unfortunately, I've never seen a motherboard or internal sound card that shipped with XLR outputs. If you want an XLR output from your pc, you'll need to buy an external USB audio interface (like the one's that steinberg or focusrite make). I personally use a Steinberg 824 with XLR outs to my amps and I couldn't be happier with it. Not cheap though.
Optical SPDIF has the advantage of creating no electrical connection between the PC and the DAC/receiver, which will also defeat ground hum and other electrical effects. The limitation of optical is that not all optical interfaces can handle very high resolution files (only some optical connections support 192KHz recordings), and optical has limitations in terms of the number of channels it can transport (only relevant if you plan on listening to surround sound audio).
If your receiver/pre-pro supports it you could also consider using HDMI out; most modern PC motherboards have one. It's a digital connection with much higher bandwidth than an optical connection. It creates an electrical connection, but it's digital so less likely to affect the output. I've never personally experienced hum with an HDMI connection.
You should check out this thread by mojave on methods of getting audio out of a computer:
http://yabb.jriver.com/interact/index.php?topic=85141.0