DAC has everything to do with sound quality and nothing to do with acceleration and rendering.Again it depends on the DAC of the sound system you use. For me going from motherboard S/PDIF straight to my Z5500 speaker setup was just as good, if not better, than instead using my old X-fi card to output via 3 separate analog cables to the same setup (plus it was easier just plugging in one cable instead of having to fuss with 3). I ended up not using my X-fi at all because of the bumps in DPC latency it was causing.
Yes, passing data over the S/PDIF over anything will allow you to use the probably superior dac of your receiver. That is why I stuck with HDMI for a few weeks instead of falling back to an analog 2 channel connection to my sound card.
What my post was outlying is that it is still ideal to use a sound card w/ digital connection then it is on board sound or hdmi digital connection (for gaming anyways).