Just so you know, the newer Magellen RoadMates like the 800, 3000T, 6000T, etc all run Windows CE (4.2 or 5.0). The OS is actually easily "unlocked" with a built in service menu accessed by pressing ++--+- on the device's keypad. Once you have hacked your device, you can do pretty much anything you want with it. Many Pocket PC and other Windows CE OS programs should work fine either out of the box or with a bit of hacking around with compatability dlls.
If would be interesing if the motherboard had some place where you could solder a CF slot or standard USB connector. Its also possible the HD itself is already in an internal CF slot... I'd love to see if it would be possible to connect a larger USB touchscreen or USB HD to this. If so, it would be a great base for a carputer! (TOMTOM!)
Really, this is almost as customizible as a full x86 carputer as it will run most Pocket PC software and of course there is much better Navigation software for WinCE than desktop Windows. You should be able to easily install TomTom on the Roadmates if you do not like the built in navigation program from Magellen. Best of all, you could run TomTom Navigator without the performance hits of a freaking emulator!
For more infomation on hacking your Roadmate and some screenshots, check out this thread here: