hmm, i dunno... if you have an ATI 5xxx gpu (or any GPU capable of HDMI passthrough/audio over HDMI) you can just use the HDMI port on the gpu to send video and audio, which would be the simplest and best way.
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my brother for some reason thinks optical is better than analog but i'm not completely sure that's true.
the vga like output is a D_SUB (and the sound card probably came with the cable) but i dunno if its ever used for audio... kinda weird how its on the sound card. From the item link,
http://www.newegg.com/Product/Produc...82E16882120187
The onkyo does have a vga input but its probably for video? (unsure) try looking it up to see if it can process that as sound.
but you can most likely use the optical/digital out from the eclaro to the digital/optical in on the onkyo.
edit: sorry i thought you had the s9400thx heh, so ignore this segment unless it still applies
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still, I would go the HDMI route if possible. If you don't have a decent HTPC GPU and live in the US, the xfx 5450 is 35$-15MIR +$6 shipping + tax at newegg
http://www.newegg.com/Product/Produc...-544-_-Product