Hi Jean Andre,
I just checked the entire Southern Tier route with Garmin's City Select version 6 detailed street maps and auto-routing data. It requires 79MB of map storage, which fits well in the Garmin 76C, 76CS, 76CSx, and 60CSx. I have heard that version 7 is built with larger map segments, which will require more space. Not a problem with the newer "x" models, but needs checking if you choose the 76C or 76CS and those maps. If you want topography too, there is plenty of room to add US Topo maps.
The route comprises 1989 waypoints, 1261 of which mark the route and 728 identify points of interest. I have not seen a consumer receiver that will hold all of these at once. The "x" models can, I read, store tracks on the removable SD cards, but I have not read about them storing waypoints. Do check this.
If I were setting out, I would consider two ways to deal with the limit of 1000 waypoints.
1. Store the data on a CD or a thumb drive with a little program like G7ToWin that can run from the CD. Find a computer shop or a friendly cyclist about half way along to reload your receiver through your USB cable.
2. Keep the POI waypoints and a few route waypoints. Every few days, spend a few minutes with the ACA maps and mark your own waypoints into the receiver for routing. You will not need anywhere near as many as the database supplies because autorouting announces the turns for you. Just enough waypoints to tell it which roads to take should be enough. From two to six per day of riding, I'd say, based on my experience doing this in New England.Where are they available for purchase?
Any of the large outdoor stores, both brick-and-mortar and on line, also many GPS specialty stores, mostly on line, where you will get more knowledgeable assistance if you have questions. An excellent list of these, along with articles about using GPS and pointers to everything, is at http://gpsinformation.net/
My own favorite vendor is tvnav.com, and many others get good press in the newsgroups.Does the gps record altitude? Is there a model that holds a internal charge so not to loose any data when changing batteries?
All of the models in the price range you need for this much storage do record altitude with each waypoint and each point in their track logs. GPS altitudes are accurate to about +-100 feet. The "S" models add a barometric altimeter for accuracy (when calibrated) to about +-30 feet.
All these models use flash memory. No battery needed to hold their data while the main batteries are changed.