I love this, CoolRed is asking for a nice simple card that only needs to be able to play multiple sounds and some people are suggesting cards that cost over 100 dollars! Every new card out today can do this feature.
You can get a Hercules Muse XL for 15 bucks or a Fortissimo II for 31. I'd get the Muse if I were in your boat. It plays multiple sounds, if that is all your looking for, then there is no need for spending more money for stuff you will never need.
You may be able to disable the onboard sound in the bios. If worse comes to worse, I believe you could just disable it in windows. Anyone correct me if I'm wrong, but I do believe you could just disable it in the device manager before you install the new card.
Only do that if you can't figure out how to disable it by another way.