I sprang for the navigation when I ordered my M3 some years back, and always felt like it was money wasted. My wife has a Garmin Street Pilot c320 portable in her car. It of course would have to be stuck somewhere in your cockpit as opposed to being mounted in the dash, but it's darn nice and $500.
If you reall want the
creme de la creme consider this aftermarket Eclipse double-din dash unit, same unit as used by Lexus:
http://www.eclipse-web.com/avn/avn5435.html
http://www.crutchfield.com/S-CwQtElM5Klg/cgi-bin/ProdView.asp?wm=fp&I=099AVN5435&g=0
tmc