Comment An easier solution (Score 1) 208
What about a netbook with a built-in mobile broadband card? These can be had for $400 or less (probably much less if you can get the mobile provider to subsidize the device). Add a decent GPS software package that includes a USB receiver and you've got a ~$500 GPS that can do it all with no hacking required.
Hell, a high-end GPS unit with half the screen size will set you back the same. But with this setup, you can take it out of the car when you get to where you are going and have a handy little netbook.
Hell, a high-end GPS unit with half the screen size will set you back the same. But with this setup, you can take it out of the car when you get to where you are going and have a handy little netbook.