My soundcard is far better than the onboard (riser card in this instance) sound, and by a good long way too. If I had never tried my new soundcard I would never have realised just how poor the onboard is, but at the same time I didn't know my onboard was poor until I put the new soundcard in.
If you're happy with your current set up then you definitely don't need a sound card. If you're sat there thinking "could I possibly get better sound than what I have now?" then I'd say you have to get one just to satisfy your own curiosity if nothing else. If you do get one though, and it's a good one, you will be glad you bought it and you'll find you never go back to onboard sound again.
If you don't use your PC for that much, maybe the odd bit of music here and there, then I'd say spend your cash on something else. If you do use your PC for audio, gaming, movies, etc then definitely get one as it makes everything just that bit better.
Also, I'm presuming your headset and speakers are decent enough quality to actually benefit from a dedicated soundcard. Too many people have a good souncard but poor speakers so they don't really see a great deal of benefit.
If you nodded off half way through that (I might have done) then I forgive you in advance
