Another computer builder

I have an HDTV setup as my second monitor. My wife watches recorded TV shows on it in the next room, and sometimes I would like to game or browse internet on my primary computer monitor. I can do this, however my sounds come through both sets of 5.1 speakers because I only have only one sound card. I know many applications such as skype can use external sound cards so you won't affect your primary sounds coming from the speakers. Didn't see an option in Media Center to do this, but I don't have a second sound card to choose from yet. And I don't want Media Center to use the default sound card because I want all other sounds to come out of it. I want only Media Center to be set on a usb sound card.