You are getting sound as good quality as that receiver via HDMI since the receiver is doing all the processing.   You do not need a sound card for that and getting a sound card only to connect it via optical out is only a down grade due to the limitation of optical.

baneling2
so you're saying the receiver is creating the sound, not the PC?

To be more technical, the source audio "creates" the sound; the receiver decodes an audio signal passed to it from the PC, and converts it to an analog signal before passing it to your speakers. There are some receivers that have analog inputs, but there aren't many left these days.
 
As such, you're discussing two different digital output options in HDMI and S/PDIF.  The audio card would be more relevant if the DAC on the card was used prior to the signal going out to the receiver, but it isn't.  As such, the receiver will decode the audio from the incoming digital signal and convert it to go to your speakers.  HDMI offers up to 7.1 audio, whereas S/PDIF only has enough bandwidth to support compressed 5.1 channel audio.  By and large, HDMI is going to be the better option in most cases for your audio situation.