If you want to take 5 bookshelf speakers and make a comp sound system with them there isn't really a good cheap way of doing it. The good way would be getting an amp with digital spdif inputs and a soundcard that will encode to digital 5.1 over spdif. Or have a buttload of wires running from your soundcard to the amp for an analog connection.
A slightly cheaper route might be to do something like what I did. Get a high end computer speaker system and replace its stock speakers with bookshelf speakers. I replaced the stock speakers on my Klipsch promedia 5.1 system with boston acoustic speakers.
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