It will fit in one of the white PCI slots.
It is not mandatory to disable the onboard. Install the audio driver (you can find it here http://support.asus.com/Download.aspx?SLanguage=en&m=Xonar%20D1&p=21&ft=4&os=29#D1_7_12_8_1794_Win7.zip
and in Windows, Sound, Playback devices set as default the ASUS audio card.
EDIT Now you should be confused.
The interface is PCI, so use one of the white slots. It's funny tough.