Just before Peter van Es came and we drove down to PUB, I had obtained a Magellan 4350 nav system at an astonishingly good price of $169 from Radio Shack in the USA.
I did this for two reasons, which many of you might understand. First, it has a build in MP3 player, so put your tunes on an SD card (Peter, I got it working, I do not know what we were doing wrong!) and to heck with an iPod. Second, and this is quite important, is the unit has an FM modulator, so all the sounds that the unit makes, including the MP3 playing, can play right through the car's radio without issue.
This latter feature is important for top down driving, as you can't hear a bloody thing from the unit itself, and I suggest the same is true from any similar nav system. Through the FM radio however, you can.
The unit is full of the latest features (3D maps, turn by turn, traffic, etc.) and I think was quite a deal. But the FM modulation is the key reason I bought it and it works well for that. It also has bluetooth handsfree for cell phones, but I don't think you could effectively use it for top-down driving. It did work OK for closed car driving. I understand it sounds like you are inside a tincan but so do most handsfree systems.