With an I/O module, the X-Fi can do digital input.
If you're looking to replace your current speakers, you're pretty limited for your options. I think the most viable solutions for you would be either Logitech z-680s or Logitech z-5500s. The Promedias don't have a digital input and neither do the gigaworks if I remember correctly.
There is also the Klipsch gmx-d set, which I've seen for relatively cheap in the past, but it's not as nice as the promedia set and you're pretty screwed for hooking up a computer to it unless you have a DDL / DTS card.
You've also got crappy HTIB as a cheap option. I wouldn't recommend it, but it's out there if you want to do it. As long as it includes an optical input on its receiver, the set would function for you. Again, like the Klipsch GMX set, you'd have issues hooking up to a standard soundcard.
Another option which I again am not too crazy about is creative makes a digital decoder box.
linky
This would allow you to take digital inputs and hook up your existing 5.1 speakers to it. I'm not crazy about this option because it's fairly hard to find these anymore and I don't like the idea of pumping this amount of money into getting more functionality into a set of budget computer speakers.