sound card for new imac

hello all, been creeping around the forums for a while. Finally decided to pull the plug and buy a brand new 27 inch imac. Only issue i know ill have once i receive it is playing surround sound through my 5.1 computer speakers. i have found the diamond extremesound 7.1 that seems to work, as well as the discontinued griffin firewave. I was just wondering if anyone has gone through this? and if any of this will be compatible with my new imacs os?

What type of input do you need for the speakers - your Mac will already put out 5.1 from a 5.1 source via optical & mdp.

No Dolby, so no faux 5.1 from stereo sources like most of your music would be.
If you have powered speakers with dolby, still don't need another piece of hardware if thy have optical input.

well it has the three inputs the pink/green/black one.