The z-5300 doesn't come with a hardware Dolby Digital or DTS decoder, and they don't have a digital input, so they aren't 'digital' speakers.
However, for ur purposes a 'digital' speaker set wouldn't be useful. Digital speakers are just speakers that have the ability to do the Digital to Analog conversion, and if they come with a decoder, they can decode Dolby Digital and/or DTS. If you're gonna be hooking them up to the computer with an Audigy 2 ZS, you already have the software capability to decode Dolby Digital or DTS, so a hardware decoder would not be necessary. Also, for PC multimedia speakers, the quality of the Digital to Analog conversion will be worse then the quality of the conversion on the Audigy 2 ZS, so you wouldn't benefit from a digital set. An example of when a digital set would be advantagous would be if you had say a Live! 5.1 soundcard, then digital speakers might be worth it, since the analog output on that card is crap compared to the ZS. For speaker sets with digital decoders, you'll be paying for a feature you'll likely never have to use, and the cost of adding the decoder probably means the speakers themselves are of lower quality then a similarly priced speaker set without a decoder.
From the three speakers listed, I'd probably get the Z-5300 but I haven't heard the GD580. Best to listen to all of them to decide which you like the best.
