The Klipsche Pro Media 5.1 is the set I wanted, but they are long discontinued. You can MAYBE find used ones on ebay. The 2.1 are still being made though.
The market for 5.1 systems for computers is pretty sad, and that is why I looked at unpowered speaker sets.
I have heard good things about the newer Logitec stuff, but my last system was a 7.1 system from them, and I was unhappy with it from the first moment I used them (I special ordered it, so I couldn't really return it). Honestly, the Klipsche 2.1 system trumps anything Logitec has, and any 5.1 stuff is just not any good (at least they stuff I have looked into).
If you really want a 5.1 setup I would really recommend going with unpowerred speakers and a receiver, simply for the speaker placement calibration features on the Yamaha receivers.
You do NOT need a U shaped desk with the Yamaha "virtual surround front", software build into the receiver. I literally have my front left and right speakers raise to ear level and angled in to face me, and my surround left and right on the edges of my desk at lower chest level pointed straight out (not directly facing me as this is best for surround channels). The software gives me a better surround experience than I have EVER had with other PC 5.1 systems (I must stress PC 5.1 systems, as I am sure a properly set up home theatre will still have much improved surround sound).
Anyway, my virtual cinema front setup puts the sound in all the right places, and I can easily tell where the sounds are coming from. The only slight discrepancy, is that the (artificially created illusion of the) speakers sound like they are about a foot away from my head at about 45 degrees behind and out to the sides (generally you want to setup surround speakers so you can't really identify where they are. This is a pretty minor thing though, and I don't really notice it while immersed in a game. I just mention it for completeness.
Here is the receiver I have: http://usa.yamaha.com/products/audio-visual/av-receivers-amps/rx/rx-v377_black_u/ . It is in the lowest end of 5.1 receivers, so as cheap as they get. You can probably get one for $250. Also they may be discontinued, or close to that, so keep an eye out. Other receivers such as anythin from Denon could be good, it is just that I can only vouch for the surround sound placement features on the receiver I have working great for a desktop setup.
**I do give this a great review, and it works great, but any dissatisfaction will come from how much room you have in your setup (let alone maxing out your $500 budget, or even pushing a little bit past). I have a large desk I made in grade 10 with a shelf built onto the back which gives me extra room. I have the receiver and centre channel under the shelf, front left/right channels on top of the shelf, and the surround channels at either end of the front of the desk. A regular desk may need a Monitor arm to make room.
Also you could have issues if you don't use HDMI to connect your monitor to your video card, since you would end up creating a 'ghost' monitor, but that problem can be worked around.