Chuck:
Look at that Matrix 97 link above. It is what you're looking for I think. Takes the little cards (the ones you want). What you said are exactly the same reasons I bought the thing. The basemap is down to 4nm (instead of 32 like the rest), so is reasonably good no matter where you go in the entire U.S., using the basemap alone.
I was originally going with a Garmin 172 and 240 Blue for depth. Picked up the 240 Blue, but they didn't have the 172 in stock. While sorting all that out and looking at the wiring harness for the 240, I started playing with this Matrix which they'd just got in and fired up. It's good, crisp big screens, and when splitting depth/charplotter they're still big. The fishfinding part of it is great. So when all was said and done I ended up with a large screen color chartplotter/depthsounder that takes the cards for slightly less than $850. Has updates via computer links with Humminbird that I haven't tried/needed just yet (since it's new), but as I am guessing, at some point it will be compatible with radar.
I have mounted mine as you said, for the same reasons. Everyone can see it from its location, and if standing in the center isle or sitting, the buttons are easilly reached for all functions of the unit. I'll probably have some pics up by the end of the day.