You won't be able to use iDVD to burn to an external DVD drive, but you can use the other iApps as usual. You'll also need to get something like Toast 6 to burn your iMovies, photos, music, etc. to DVD on the external drive, though. (It will let you do basic menus, chapters, etc ... just not as nice-looking as iDVD.)
FCE is just for making movies, as far as I know. If you want to use Apple-branded software to burn DVDs on an external drive, I think your only choice is DVD Studio Pro.