And now some hints about using your 478 for the TAT.
I'm assuming you've already purchased the maps from SAM. Its a touchy subject here as far as sharing the GPS tracks for the TAT, but if you prove you've spent the money for the maps for what ever part of the TAT you are doing, I'm sure you can find people to share their tracks with you since you bought the maps. However, from experience, I can tell you that sitting down and plotting out the tracks yourself is well worth your time. You learn a lot about the TAT just from reading the roll charts and following the maps.
Most people find that plotting "tracks" to follow on the TAT is much more beneficial then "routes". A track is just basically a "bread crumb trail" you are following on the GPS. You won't get any turn by turn directions, but its easy to follow it on the screen. You can actually do both and is what I did. I liked having the turn by turn directions routes give you, but I always a track as a backup just incase the route did something weird. The reason being is that routes always have to follow roads, but the tracks don't. I think for the most part, the Eastern TAT, you won't have any problems following roads that exist in Mapsource, but out West, it gets really interesting I've heard. I've done from Jellico, TN to the OK border myself.
Your 478 will support 15 saved tracks of 700 pts each along with its active log of 10,000 pts. It will also support about 50 routes with a certain number of waypoints. (can't recall off my head). Also it will support numerous "via" points inside a route which do differ from waypoints. Learn the difference.
You need to spend some time learning how to force MapSource to route the roads you want by inserting via points in strategic places and using Waypoints for important points of interest.
A good program to add to your PC is called GPSbabel
. It will convert routes to tracks and vice versa, slim down a track to a certain number of points etc. It was invaluable to me when creating my TAT tracks.
I hope this all helps.