Another possibility, if you already own a Palm or PocketPC, is to buy a receiver and connect it to the PDA.
Here are two that use a Compact Flash slot, but there are also cabled and Bluetooth devices out there.
[www.geekzone.co.nz]
[www.pocketgpsworld.com]
If you do go the PDA route, you'll need software. I highly recommend DeLorme's Street Atlas USA Handheld. You can download only the data you need, and create your own custom routes and map segments (see image below):
[www.delorme.com]
Delorme also sells a Bluetooth GPS unit:
[www.delorme.com]
JAC