I suggest you also take a look on Creative Gigaworks S750. I saw a review of those vs Z-5500 and the best Klipsch PC system from a few years ago and Creative's set was regarded as better than Z-5500. I'd post a link but can't find it anymore, sry. They only have analog connection, but it shouldn't concern you as you only intend to use them with your PC, and X-Fi I believe offers quite good sonic qualities.
The good thing about them is that their satellites have binding posts so you can use them with any amplifier or receiver. I actually use them nowadays with my Denon 2308 (until I get some money to replace them with a more serious system) with them amplified by the receiver, and the sub connected to the receiver's sub out.
Some people have said they had problems with the built-in amplifier failing after a year or two, but mine are 4 years old and still function flawlessly. All the speakers are actually manufactured by Cambridge Soundworks (I opened them up and only Cambridge Soundworks is written on the magnets, no mention of Creative there), Creative owns that company so they only put their name on.
There is a used set on amazon.com going for 250$. Maybe it's worth taking a look at:
http://www.amazon.com/Creative-Labs-Gigaworks-Speaker-System/dp/B00013JU9S/ref=sr_1_1?ie=UTF8&s=electronics&qid=1260188230&sr=8-1
edit: I have also heard Z-2300 that I believe shares the satellites with the Z-5500 system and the Gigaworks satellites, in my opinion, offer better quality; they are 2-way while Logitech's are a single full-range driver, that probably helps somewhat