I'm not really familiar with those Cyber Acoustics speakers, but I don't see anything about them having a digital coaxial or digital optical input. If they really have a digital input, they're being incredibly vague about it, and if I had to place a bet, I would say they don't actually have a digital input.
There's always going to be an analog conversion before the sound gets to your ears. All speakers are analog, even "digital speakers". If you get a set with a digital input (like Logitech z-5500 / z-5450 / z-680, Klipsch GMX-D, etc.), then all you're doing is instead of your soundcard doing the conversion, it's the little control pod attached to the speakers that do the conversion.
A good quality soundcard is going to do a better job than the control pods these speaker systems have. You're not really gaining anything quality wise having your speakers do the decoding and conversion vs a good soundcard.
All these "digital speakers" pretty much do the same thing. "Digital ready" speakers can pretty much all decode DD (and some DTS) streams and give you 5.1
If you get a set of 5.1 speakers with a digital input, the only way you're going to get them to all play is if they can decode a DD / DTS stream or if they try to fake it with some sort of surround mode (like dolby prologic II for example).
If you're doing MP3 / CD / DVD sources, you don't need a card that can encode DD / DTS, you just need a card that can decode it. Pretty much every card and integrated solution can do this.
MP3 / CD can be sent to your speakers through analog stereo or a PCM digital signal. A decent soundcard will probably get you better results with its analog output than sending digital to one of these "digital speaker" sets and having it do the conversion.
For DVD, you can have your soundcard output the analog 5.1 signal already decoded or you can send the DD / DTS stream digitally and then have a control pod do the same thing your soundcard would have done.
The end product of sending digital out is pretty much just having your speakers do what your soundcard would have done already. A digital speaker set will probably do a better job than your average integrated solution, but if you're planning on buying a decent soundcard then you're probably better off using its analog output.
If your motherboard has digital output, then this might be a good idea. Digital output from your motherboard should in theory be as good as the digital output from any other card for PCM signals and DD / DTS passthrough. If you want to use your integrated digital sound, then getting a set of speakers with digital inputs makes sense.
The only time you'd run into trouble with this is if you wanted to do games in surround sound. This is when you'd want a card like the X-Mystique. The only time you need a card that can encode DD / DTS is when you're playing a game. These cards will take what would have been an analog EAX 2.0 output and encode it into DD / DTS to be decoded. The sound gets compressed a bit like any DD / DTS stream in the process. You would probably get better results running analog output from a creative card for gaming.
I think the only reason to really buy one of these DDL / DTS encoding cards is when you want to hook up to a receiver / pre-pro and don't want to hook up a 5.1 analog connection due to a variety of reasons. For a situation where you're going to be using computer speakers I think you'd be in better shape if you just stuck with analog. I think this whole "digital speakers" thing is a great example of "digital" not always being better.
I don't know if any of this makes sense to you or if I even did a decent job writing it. I just got back from a week vacation and I'm a bit out of it.
I think some good options for you to consider might be
integrated digital + Logitech z-5500s
midrange analog card (A2 / whatever) + analog input speakers of your choice that you can afford
EDIT: and don't get the Logitech 5450s. They're basically like the z-5300s at triple the price and you get a few "features" that I consider to be pretty worthless. If you're getting a logitech set, get the 5300s or the 5500s.