Why not just get the best of both worlds and purchase a PDA and install GPS mapping software on it. Then you can have both your Empeg and your GPS navigation. Trust me, you will use your Empeg more!
There are two products available, which I have both. You can private message me if you want to know the pro's and con's of either.
Check them out at:
http://www.travroute.com for Pocket Co-Pilot or
http://www.destinator1.com for destinator.
The Desintator uses Navtech maps, which are the same the Mercedes uses from what I remember reading.
You can check out my setup at
http://www.geocities.com/neo35faq
Jarred