I utlize the same setup to use the usb headset for voice operations and my soundcard for game, movie, etc. So what I do is run my usb headset for Teamspeak and have the game run on my soundcard. Anyhow:
Windows Xp option:
Start--> Control Panel--> Sounds,Speech, and Audio Devices-->Sounds and Audio-->(properties window will pull up)--> You will then be able to pick the Audio or Voice tab and set your default devices for what you want.
In my setup I have audio set to my soundcard and voice playback and recording to my usb headset.
Hope this helps