Raggie,
Go here for all GPS all the time (information):
GSP Information
Several questions to ask... Price range, what type of usage (driving, hiking, both). And to connect with/use a computer for trip planing and such...
A couple to look at:
NUVI 350 (recommended to elder woman--loves it--car only, no maps for computer, supports MP3--most expensive)
Street Pilot 2720 (full size unit--car only, with maps for computer)
GPS 60C(x)x (hand-held protable--hike/car--no voice--maps for computer) The 60Cx and 60Csx is a new design with extra memory card and VERY Sensitive GPS receiver)
GPS 76(x)x (about the same as 60C(x)x but slightly larger to float--buttons in different location)
The above are a good place to start and see what interests your Dad (for car navigation)... There are other brands (like Magellan, Lowrance) that also may have units that may be better for your Dad's particular requirments.
Like most things, much of the software for a computer is for Windows (some others are available--look before buying). Also, look at the maps that you are interested in... Each brand supports only its own maps.
Good luck.
-Bill