I would get an Audigy 2 for that much money.
I don't think you're going to noitce any major differences with speakers for under $100.00. If you want more Bass, then maybe Logitech z560. those are around 125 to 150 depending deal of the day.
Even with an Audigy 2, you won't notice everyday differences for mp3s etc. In some games, you might be able to tell etc.