I want to get a gps device for hiking, but Im not sure what kind to get. and under $400

I am looking at cars, and it seems like if you want a GPS navigation system it costs thousands more, but if you go to circuit city or somewhere where they sell those hand-help GPS navigation units like from Garmin etc. they cost only a couple hundred. Is there a difference between these two that I can’t see which explains why the car navigation systems cost so much more?

I am fairly new to the Geocaching Scene and would like to hide a few of my own but I have lost the Manual that goes with my Hand held GPS and need to figure out how to find the correct cordinates.

Are the 14, or 16’s able to stay locked on to GPS signals better without loss of the signal?

I’m going on a long flight pretty soon and I have a handheld Garmin GPS. I’d like to use it to see where I am, how fast we’re going, stuff like that. Is it safe to turn a GPS on when flying? What about during takeoff and landing?