As stated above RetroPie is already running on top of Raspbian.
If you’re looking to have a dual purpose retro gaming setup and media center there are existing solutions that you may want to consider:
http://www.raspberrypi.org/forums/viewtopic.php?f=78&t=59590
http://www.recalbox.com/
There are also ways to modify RetroPie to support launching into XBMC but it would take some work.