On import you choose whether to have Aperture copy the files into its database (Managed Library) or you choose to have it work with the files where they are (Referenced Library). I use a Referenced Library because it keeps the size of the database to a manageable size. In a Reference Library, the Aperture database contains the settings, metadata, etc but only a link to the images. I see this as the only viable method for D800 shooters. Using a Managed Library for those large files can and will cause that database to grow very, very large.
I dug through my bookmarks and found this article from Macworld that I believe I read and used to make my decision:http://www.macworld.com/article/1161525/understanding_aperture_importing.html