Make model of the receiver? something dont sound right might need to clarify your question... not sure how you taking a PC 'one pin' to Aux in. are we talking RCA analog or a Digital output from the PC?
if you are running digital output from the PC, then SPDIF will not change anything as its the same data just a different transmission method. optical vs digital over wire. actually you might end up having less latency with a non-optical signal as there is a tiny delay when the data has to be converted. I say this cuz I hear it when I monitor my traktor vs hearing it come out of my home system.
Oh and if you arent using higher then 1080p or if you are using HDMI 2.x then I would use that to connect your PC to your receiver for your 5.1 then the special encoding card isnt needed as hdmi gives discreet channels and would eliminate the need for encoding.
If your connection to your receiver is analog then yes you will see a improvement by going digital transmission.
Key point is you need to clarify what receiver you have and exactly what you mean by 'one pin', because your question has some contridictions in what you doing.
Based on what you said you are already using digital but the whole AUX in of music system(two pins) doesnt make sense at all unless its analog...
If you are using digital you dont need to change anything other then get a soundcard that is designed for 'encoding' discreet PC channels over the 2channel digital connection. Digital out/SPDIF is not discreet multichannel you have to encode it. Anything that supports Dolby Digital Live or DTS-Connect is what you will need if this is ultimately your question/problem.