Using regular computer speakers with a console is a terrible idea IMO. Without using the optical out on your console you won't get "true" 5.1.
I have a Creative Inspire GD580... I don't think they make it anymore. I liked it, but after being on for a while there would be static. I figure the decoder would get hot or something.
Now I use the Pioneer HTS-GS1 that I got from Buy.com. It is "styled" for the 360, but works well. I'm even more happy with it, and use it connected to my cable box for audio as well.
Each system cost me $100, which I felt was a good value. Of course they don't compete with high-end speakers and receivers, but they certainly do the job if you're on a budget.