VERY few sound cards, outside of those aimed at professionals, have any kind of optical or coaxial INPUTS. It's far more common for them to have optical and/or coaxial OUTPUTS. I would absolutely double check that those are indeed input ports. And actually go and check the documentation and search on Google, not just say that it is because you think you read it somewhere.
That all being said, unless Microsoft dramatically improved the audio processor on the 360 over the original Xbox, the 5.1 audio is not worth the bother. It promises the world, and delivers a small third world country. It's just upmixed Pro Logic II (stereo + LFE) or stereo, and it tends to drown out the center channel which is used for vocals. Given the amount of space that would be required on a game disc for discreet 5.1 channeling, it's only going to happen on PlayStation 3 exclusive titles on Blu-Ray discs for the foreseeable future. If Microsoft releases a new version of the 360 with the HD-DVD drive replacing the standard DVD drive it has now... Maybe. Not that it would do you any good unless you bought a new unit.
Even with an after market receiver, I don't really see this working out like you're imagining it. I'd save my money if I were you. Or if you do get an after market receiver, make sure you have some uses for it other than your 360.
Hey there, just looking for some help. I have a Nforce 3 (MSI K8N Neo) and it has Realtek 97' onboard audio which support 7.1 audio along with optical input and coaxial input also. I have Z5300, and i have my Xbox 360 connected to my 21' inch LCD Samsung (widescreen) via VGA cable. I would like to use 5.1 audio, and was wondering if i could go about getting an optical cable, connecting it to the Xbox 360 and running it to my Mobo and letting my soundcard doing the emulation and bouncing it back to my Xbox 360. My PC should be able to handle it if its CPU intensive (Athlon 64, 3400+, 4GB Ram). Let me know if this setup will work or if i will need to invest in an aftermarket reciever.