It seems most handheld GPS can be connected to a computer (through a
serial
cable, IIRC).
That allows some cool features, like preparing your flightplan in advance
on
your PC, or downloading to the PC the actual positions of your aircraft in
order to obtain a chart of the actual flight path.
Now, do you know if panel-mounted GPS can also be connected to a PC ?
Namely
for Garmin's GNS 530 ?

The 430/530 has a serial port output. If you wire the aircraft for it, then
you could connect it to your PDA or notebook to get a moving map, which
makes no sense because the 530 is already a moving map. Plus the data
format is King/Aviation, not NMEA, so most off the shelf moving map programs
couldn't interpret the serial data anyway. One interesting thing you could
do is to capture and store the flight data so you can look at it later. No
normal 430/530 installation puts a serial port connection there for you on
the panel, it would have to be a special thing for your airplane.

The 430/530 has several serial ports but they are for connecting to other
equipment or a 2nd 530 unit for cross filling flight plans. The cross fill
serial port details are not published so you can't upload your own flight
plan from your PDA or laptop either.

I can't see any reference to this on the 530 reference manual, and I would
be surprise that handheld GPS can do thing that Garmin's top tier GPS
can't.

It's a certification thing. Nobody wants you to be able to load a flight
plan from an external source. There is no assurance that the data is valid.
No general aviation panel mounted TSO'd GPS will allow you to externally
load a flight plan. All of them will provide serial data output in various
formats but are rarely wired for the pilot to have access.

If you want to play with a GPS in this manner, use your handheld.