How can I listen to dolby digital 5.1 sound on my RS404 ?

[jpvigno]

I just bought a new RS404 and I wonder what is the best solution for listening to dolby 5.1 sound when watching DVDs :
I planned to buy an external USB sound card such as Audigy or Extigy (by the way what is the difference?), connect it to the optical SPDIF connector on the back of my PC, and to a speakers system such as Altec Lansing 251 (I guess in that case I do not need the digital version of it, simply the standard version with 3.5 jack connectors).
Is it correct ?
Or is the Digital version of the Altec Lansing enough ? Or another 5.1 kit ? Can I connect it to the optical SPDIF connector ?
Thanks for helping !

[intradinkq]

If the pc has an SPDIF sockect on the back, it would suggest that it already supports digital sound. You'll need either 5.1 speakers (with built in amp) to hook it up, or plug the SPDIF into a 5.1 amp with speakers connected to that.

Hope that makes sense