be prepared that a atom/zacate pc is going to be a little sluggish with centrafuse-- it uses the processor for everything.. most that want a snappy pc will look to using a core2duo, or i3 to keep a snappy interface.
i really have nothing against msi, i am not crazy about gigabyte only because they like to change drivers in board revisions, and it is a headache for my job(a moot point when your only buying one board...), so i end up recommending zotac, as they seem to be a off-shoot of asus, and have always had very few problems with asus/zotac stuff.
for the ram, i don't think any more then 2gb is needed--unless you plan on using the pc for multizone, and then the processor comes into question...
for the audio questions-- it is easiest to just connect to some aftermarket amps using some 3.5mm-to-rca adapters.
the homemade amps, are you talking about builds like nivanov's?
i don't use radio, so can't comment on that..
dac is litterally a sound card-- a high end sound card. most do not feel the need for it, and usually just use the onboard outputs, but it is personal preference..