It really depends on the quality of speakers you have.  SBLive isn't a bad card at all.  Do you have surround sound speakers?  
Unless you've got a really good speaker system, you won't notice the difference, most likely.   If that money is really burning a hole in your pocket, and you want to spend it on your computer sound, I'd make sure I had the speakers first.  You can get a really good set at a wholesaler or online for cheap.