If you got the money for a PowerMac G5 and really want some great sound. There's nothing better than a decent to quality 5.1 -7.1 reciever w/remote /speakers and a Toslink connection. Hook up everything, control it with the remote.
Surround Sound cards just plain sucubus in comparision.
Make sure the reciever can decode most of the surround sound formats so you can watch DVD movies.
I also suggest getting this $15 piece of software, it will do amazing things to your iTunes music as well as level the volume from song to song.
http://www.volumelogic.com/index.asp?content=VolumeLogicPlugins2
(I recieve no comp, I just think this is about the best $15 I've spent on audio software)