ZX's analogue out or coax out, or MoBo coax out?

[eatmuchpie]

Hello, this is my first post, so please be kind

I'm currently speccing a new computer and have got just one thing to work out. I have a creative inspire 5.1 digital 5500 (basically 5.1 speakers with a coax/optical decoder). I want to connect my Xbox to it (through optical), so if I get a coax output from my computer to the decoder, then all will be well and to switch between xbox and PC sound all I'd do is press a button. However, on my new motherboard (Asus A8n SLI Premium), there is a coax out, but I have the money to buy an audigy 2 ZX (with a funky breakout box).

My question is will I notice any difference in sound quality if I use the ZX? (I'll be using ac3jack, to stream 5.1 output via S/PDIF in linux).

Additionaly, I could use the ZX's optical input for the Xbox, then output from my computer via analogue, thus leaving out the decoder entirely.

So which method would provide the best quality?

Thanks for any help!

Edit: Because I'll be mostly using it in linux, I have no need for EAX.

 

[Psypher5]

I had an audigy2, used analogue digital out, was a lot better than a8n onboard digital im using now (current speakers are logi's z-5500 - untill I get an xfi Fatality or an HDA to really let them shine)

Personally I'd go for the audigy.