for what its worth, I use the Garmin 60C which is smaller than what you are looking for but also serves a dual purpose for off road navigation AND take it with you for hiking/biking/geocaching.
I dont have any street maps loaded on it other than the base map that comes with it. So I just load the USGS maps for backroads. I use a Pioneer in-dash Nav system for streets and businesses when necessary, or the Blackberry with TeleNav (sad but true, I do travel with 3 GPS devices in the van sometimes). But the 60C and the newer CS is a neat little GPS, color screen, very capable. Available on Amazon, REI, any good outdoor store. Here's how I have it mounted in the SMB, top left side of the dash and almost eye level but safely out of the direct line of vision when driving: