I've had good luck with Logitech 2.1 speaker set ups, Altec-Lansing also has some decent 2.1 set ups. For strictly music I prefer the less cluttered 2.1 sets instead of having a surround set up and wires everywhere.
I have an early set of Z series Logitech 2.1s that were plenty for normal PC music listening, something similar should run around $75. I got a THX certified set since then that have plenty of power and bass. The THX sets are around $200.
I've always used the onboard sound, and the Gigabyte Z77-UD3H I currently have supports 7.1 surround and has some other fancier outputs as well. I don't think you'd get much benefit out of a sound card unless you were doing professional editing and using a fairly high end sound card and speaker set-up (which would be more than your budget I think)