Klipsch used to make a 4.2 system
http://www.klipsch.com/product...ails/promedia-4-2.aspx
This is not a good way to go for what you're trying to do.
Klipsch Satellite speakers are basically just small poor quality speakers that are 4-ohm and would present a more difficult load to the receiver if you were planning on using the receiver to power them. I don't really know how the volume control works with the sets, so I don't even know if it would really work to have the receiver try to power the speakers while sending a low level signal to just the sub.
It would be difficult to find a center channel that really matched these except for another promedia satellite/center that someone is selling separately from a complete set.
If you want to use the soundcard to control the two speaker sets and then a receiver (or I would recommend just a small amp like a T-amp) to drive the center speaker only, then it's going to be really irritating to try to adjust system volume as you'll have three different volume controls to adjust / turn on when you want to use the system.
If you wanted to use the Klipsch subs connected to the receiver with the subs powering the speakers, you'd need a receiver that has pre-outs for all channels and hook up that way (most inexpensive receiver don't have that). You'd still have to deal with three volume controls.
You can reject "real speakers" if you want to, but that the easiest way to get a system that performs well in the end and will not be an awkward mess of different systems wired together. If you want small Klipsch speakers, Klipsch makes some real ones that are small too. example:
http://www.klipsch.com/products/details/rsx-3.aspx
I don't understand why you wouldn't want to do a "real HT" type setup if you're already planning a relatively large system and a receiver's going to be involved.
If you're pricing Klipsch 2.1 sets at $150 each, that's maybe
$150+$150+$150(receiver)+$50(center?)
That's ~$500 for something similar to a Klipsch Promedia Ultra System?
If you want a surround sound Klipsch computer speaker set, get a set of Promedia Ultra's off ebay or something. (Looks like you could get a set for about $300 shipped off ebay?)