Howdy,
For what you described in your first paragraph, you could easily get by with
a Garmin Etrex. I got one (Legend) for about $140 new on ebay and am very
happy with it. It has airports, major highways, cities, some rivers and
lakes, and you can plug in plenty of your own waypoints. It won't have
airport identifiers in the database, or VOR's, etc. But it'll do what you
described.

The only time my Etrex sees aviation use is when the kids are bored in the
back of the plane. I have an Ipaq 5555 and Anywhere Map, and love it. I
have the previously mentioned wireless "bluetooth" GPS, it has WAAS, and can
move easily from car to airplane, as can the Ipaq. Each unit has their own
built in rechargeable batteries, so back up is not a problem. Like "John T"
said, there is the matter of the yoke mount, clip, sleeve, etc.
I used to own an Airmap300, and it was a fine unit also.
An IFR GPS in the panel would be ideal, but the PDA route is so adaptable
(Windows based) and I use it outside of the airplane plenty, I'd still
probably use it. Matter of fact some IFR GPS's output can go directly into
the PDA for the GPS signal. I'll probably get a Garmin GNC300XL and use it
this way, but it aint cheap.

Do you think you'd use it for street directions? If so I'd consider the
Garmin StreetPilot. Big color display, all the info you want, and it talks
to you for directions. Probably chews through the batteries though.

Good luck, Chris