You do have onboard sound which to Windows functions much the same way as a dedicated sound card. So now you'll have two. It's simply a matter of choosing which one you want to be the default device from the Windows sound preferences.
Physically installing sound cards is usually pretty easy (some come with additional front bays and what not, though which means a few more cables to connect). Plug it in the right slot, then unplug the front panel audio from the motherboard and connect it to your sound card instead.
As for the sound quality that depends. Onboard sound really isn't that bad at all. For casual use, you probably will not notice the difference. But good sound cards typically do feature better quality amps, so if you want to record things you will find a decent sound card can dramatically improve clarity of the recording simply through better volume and less noise from the amp.
Bookmarks