Using my onboard soundcard to decode Xbox SPDIF?

I've seen variants on this question but I don't think I've seen this exact one so apologies if I'm repeating.

I'm trying to use my computer as a 5.1 receiver, mostly because a long time ago I bought an analog 5.1 Soundworks speaker system that doesn't accept digital-in (so only computers can output to it). I have a Gigabyte GA-MA790X-UD4P motherboard with Realtek ALC889A decoder; I also went and bought the special SPDIF-in bracket.

I've hooked up my PS2 and XBox, and so far it can decode stereo input fine. Does anyone have any idea if/how it can support 5.1 SPDIF input?

Thanks in advance for your help!!!

Yep. 5.1 can only be transmitted over Spdif in a A3C (Dolby Digital) or DTS stream, due to bandwith limitations. Most soundcards can at least decode Dolby Digital (most can also decode DTS), as do most recievers. But no onboard chipsets I know of allow decoding of DD/DTS signals, so you are limited to 2.0 over Spdif.

Best solution would be a soundcard with a Dolby Digital decoder included. Make sure it also supports a digital input. I believe both the Auzentech Forte and ASUS Xonar D2/D2X should suit your needs. Alternativly, you could get a digital reciever, but those would almost certainly cost more.