First off what I believe you are looking for is a Dolby Digital 5.1 Speaker
System.
A 5.1
system actually consists of 6 separate speakers:
-Front Left
-Front Right
-Center Channel
-Rear (or Surround) Left
-Rear (or Surround) Right
-Subwoofer (also known as LFE channel), that's the .1
A Dolby Digital 5.1 source (such as a DVD) will produce sound from 6 separate or discrete channels which is why you
need all 6 speakers to fully enjoy what is being presented.
I don't have a 5.1 speaker system for my PC (I have that for my Home Theater) but it seems they range from about $185 - $400.
Take a look at this page on Newegg's site:
Newegg's Speaker Page
They list the Cambridge system for $185 and the Klipsch ProMedia for $400.
I have heard the Klipsch system at an in-store demo and they sounded very nice!