Hi Deltic,
Yes you can easily have a GPS with road and aviation databases and works very well.
I have been using an O2 XDA2 for the last couple of years (mobile phone/pda combination), but the system will work just as well with HP etc PDA's. Bluetooth connection linking a TomTom GPS receiver for position, then TomTom software for road directions and Memory Map for the aviation database. Memory Map has 1:250,000 and 1:500,000 aeronautical charts all available as well as OS maps if you're into walking.
I have been working part time as a taxi driver and can tell you that the TomTom works very well to find your way around the roads (including by postcode). I've not used Memory Map very much as it's only a "just in case" option for me, but I have tried it and it seems to work fine.
The best bit about it for me is that this way combines my PDA, Mobile phone and GPS (road and air) into the one unit (well 2 units really if you count the very small bluetooth gps receiver).
Hope this helps you.
Chinchilla.