james.miller is correct, digital doesn't equal better.
I myself have used the digital out (SPDIF out) on the Audigy 2 ZS with my Z-680s, and it doesn't sound a TAD bit cleaner, but not a whole lot. You'll need to get an in-line adapter (RCA to 1/8" mono) in order to use digital on the Audigys, and it does not do surround sound for music or games, only in moves. And even in movies, I found the center channel to be far to loud, even when turning it down so far without shutting it off completely. I found Stereo x2 to be better for music listening, and you have to use Analog 6 Channel Direct to achieve "true" 5.1 surround sound in games and movies.
If you REALLY want to use Dolby Digital for movies, games, music and anything else, get an X-Mystique 7.1 Gold, as it encodes all audio going into it to DD 5.1 before outputting it to your speakers (the Z-680s and Z-5500s both have built in decoders). So 2.1 MP3s will be evenly distributed to all 5 satellites, movies will sound more realistic, games that don't support Dolby Digital 5.1 will still "have" (note the quotes) the sound of DD 5.1, and your bass will definately be clearer and tighter than before.
I noticed the Audigy 2 ZS garbled my sub and it sounded very muddy compared to using the on board’s analog and digital channels. Thanks go out to Shard helping me to identify this.
If he hadn't found that fault in it (I'm a semi-audio noob), I would have never been able to really appreciate my speakers and what they can REALLY do. In my opinion, the Audigy 2 ZS and anything lower just degrades the Z-680s or Z-5500s, and you'll never really be able to hear what everyone is raving about when they say that they're the greatest PC 5.1 speaker system out there.