DeLorme Street Atlas allows you to install a version to your handheld. It lets you save map segments to your handheld (you can save as much as you have space for). It even does navigation on the handheld if it has a GPS connected.
I've messed with it a little bit on my Garmin iQue 3600 which is a Palm-based PDA with integrated GPS. Navigation works. I don't recall what the interface was like for setting up a route on the handheld. After my experience with the excellent Garmin software on my iQue I found the DeLorme piece for the PDA to be pretty gruesome but my brother does some occasional walking-around navigation with Street Atlas on his Palm Tungsten with
Bluetooth GPS receiver and he seems to like it. You might find it quite useful for your needs and Street Atlas is very affordable.
There are other options but I don't know how much money and time you want to spend and I don't know if you want to do navigation with a GPS receiver or just plan routes and have a nice map display with the route on it.
...ken...