Your sound card probably has more than one stereo output, I would think. The "line out" jack will work with powered speakers. If the speakers you have in mind are not powered, you would need to be more creative. My own thinking is that adding more speakers arbitrarily will not necessarily improve your sound but might muddy it and create some challenges getting everything balanced.
I have Cratives new set of Speakers, a 2.1 affair, Load and quite good looking too, but I Spotted a small pair of Telescopic tube speakers in this weeks Micro Mart magazine, they can be mounted on a monitor and are just so cute. I have a Minijack splitter like this http://pdf.fifonetwork.com/foto/Sandberg/502-16.jpg that I got from a cheap pair of headphones spare, is it possible to run the 2.1 Creative speakers and the Monitor speakers together using the splitter, I only have a basic onboard Realtek '97 6.1 Surround card, is it possible?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic