As long as you understand the constraints that all current PC speakers have, I'd recommend something like the Logitech Z-5300's w/in your budget. With a 5.1 PC speaker setup you are, indeed, sacrifcing something in individual speaker quality but for your intended use and budget I'm sure you're aware of this.
I remember when I was considering the Z-5500's (which have now had for a couple of months) and some told me that they were junk with low dispersion, etc. I myself even wrote a lot of negative comments about them prior to owning a set. Realistically, none of these types of speakers are going to be really really great or better than your mid-fi home theater kits. ...but the Creative GigaWorks S750 may be the contender for best multi-channel PC speakers about. ...and they seem to be built better than the Klipsch 5.1 kit. ...but they're more speakers than you need and out of your stated price range right now.
BUT...but the Z-5500's certainly are a tremendously fun sounding set for personal surround loudspeakers with way more bass than I'll ever be permitted to use in my residence. The bass can kill or neutralize bacteria on nearby walls. While I'd highly recommend listening to the Z-5500's I know they're out of your price range right now, as well. ...but it's a nice system, especially with its nicely made DD/DTS decoder.
I imagine the Z-5500's little brothers, the Z-5300's probably do quite well too, at their price, and would point you in their direction. Paired with an X-Fi or other contemporary 5.1+ audio card you'll likely be in for tremendous audio enjoyment. I think if you go lower than the Z-5300's you'll shortly want to upgrade.