Garmin GPS data, options and costs

[jephotog]

I am looking at buying a GPS and the Garmin 378 has become available for $400. It is just what I am looking for in a GPS in terms of:

Battery powered
Portable from boat to boat to car
Fish finder option
XM option

My only concern is the database in it is the inland lakes database which does not actually include the great lakes. It also includes the US city and roads database. Since I live inland it does not seem like a bad option, even though I have heard it does not include all lakes and you do not know which ones it does until you own it.

Garmin has a number of different mapping level technology does any one know which type of maps are in this database, is it the mapsource?

I know that the 378 can accept the G2 data but not the G2 vision. Both of these seem cost prohibitive for me at this point. My concern is to buy a $400 GPS then have to spend thousands to get it filled with charts. Does anyone know of my other options for chart data for when I visit the coasts or should I just pay the extra to get the 478, which comes with coastal maps installed instead of the inland waterways?