This may be confusing, let me know.
When you use the 3 headphone jacks to connet audio from PC to the Receiver, it is analog. The sound card is doing the work needed to make the sound use all 5.1 speakers.
When you use the optical cable, you are limited to ->
1. 2 channel LPCM audio.
2. Or, encoded 5.1 streams from DVD or file (DTS or AC-3 streams)
3. There is no Dolby digital live or whatever its called (live 5.1?), on the SPDIF or the optical cable. You would need this to play games in 5.1 while using optical.
The sound card is just passing on all the digital data it is getting from the source, its not doing the decoding.
You need a card that does dolby digital live encoding. Its on the fly 5.1 encoding to ac3 stream.