You can only select a single sound card, motherboard chip, or video card audio decoder at a time.
So you should only have a single sound card installed.
Windows XP actually had problems with two sounds cards physically installed. Vista and Windows 7 can handle it, but again there's no point since you can only use one at a time.
So basically you'll want a different sound card and make sure the onboard audio is disabled in the BIOS, and the drivers/software for the sound card are installed.
I really like my Auzentech X-Fi Forte.
*Make sure the sound card supports multiple outputs if you require them. My card supports up to 7.1 as well as an additional stereo output at the same time.