Normally you would stream audio files over the network.... However if you wish to do a direct connection you would connect via the digital optical out present on pretty much every modern motherboard ever made. Or use HDMI. Its a digital signal either way so I'm not sure I get your question about the signal to noise ratio at all? Its DIGITAL.
Sound cards make about no audible difference at all and there's really no point in one if you are sending a digital signal to an AVR and letting it decode it. However if using one you would of course use its digital optical out as well.