This is may not be what you are looking for but I use a Garmin Geko 201 for navigation and as an odometer for running and cycling. Its a low-end unit (I paid $120USD 3 years ago) but it was the least expensive one I found that has WAAS.
Its not a mapping unit but I have programmed in waypoints to find addresses in the past. It does record where you've been and can accept waypoints for navigation either from a stored position or manually entered. While it won't give you turn-by-turn directions, it at least gets me close enough to a location I am trying to find (as long as the lat/long is reasonably accurate).
I am going to try GPS Connect for OS X very soon. I just need to get a serial to USB converter for the Garmin data cable
If you have to run a Windows based navigation program, it could be done using either Bootcamp or Parallels though....