As said earlier, if you are doing standard non-commercial flying and want the GPS to help - get a Garmin or Skymap
It isn't clear from your original post what you will actually need one for. Certainly if you are training, then the plane you fly will be properly equipped. If you want it for road navigation, then get one with the road database
Alternatively, if it is just for play, then why not get one of the iPaq-based ones.
Just sounds to me as though you have a wider brief than you have asked, and no one product will fit your bill