External Sound Card - Totally confused

Hoping for a simple answer to my question. I have been reading posts and articles to get an answer and I am now confused.

I have a set of Logitech Z530 5.1 speakers. There is no optical out just the 3 3.5mm plugs (Front, Rear & Centre/Sub). I am wanting to use these with my new rMBP when it arrives.

Is there an external sound card that will provide me this functionality. I am not an Audiophile so I don't need incredible sound or heaps of EQ functions, just a solution that works.

Can anyone provide a simple (preferably low cost) solution?

Thanks

thanks for the reply, reading the comments - It does not work on LINUX. Not sure if this would be a similar result with Mac.

Any other suggestions from any that are known to work with mac?