try to find these
http://www.newegg.com/Product/Product.aspx?Item=N82E16836121122&cm_re=logitech_speakers-_-36-121-122-_-ProductThey have recently gone up 50% in price tho, I got these twice for 100 each, different PCs and are great bang for your buck.
Personally I use 5.1 Creative THX surround system built by Cambridge Sound Works combined with a creative X-Fi soundcard but I spent a pretty penny on them, and they are impossible to find now
You should get a soundcard too if you listen to a lot of mp3s on PC, if its CDs I wouldnt be too worried