Hi Peter -
A GPS is a fantastic companion to have in North America - as you say, it has points of interest, hotels, fuel stations, restaurants etc. all in there (not every one mind you, but a hell of a lot, and plenty to help you plan your routes). Also, if you get a Garmin unit, then their City Navigator mapping software also has pretty much every navigable dirt road in there too, and the 'shortest route' function will typically route you along those just as if they were tarmac - excellent!
These days, you can buy a new Garmin Nuvi 205 for around $170 USD in most chain electrical stores, perhaps less if you look on eBay stores and have an address you can use for shipping. The Nuvi is not a waterproof unit, but is very compact and could be mounted just about anywhere for protection, and has a built in battery too so you can use/program it away from the bike.
Personally I have the older style Garmin 2610 that a lot of bike travellers swear by (as do I) - it is a fantastic machine, robust, waterproof and very easy to use on the move. The only downside is it has no battery, so you need the mains adapter if using it in hotel etc. Also, the maps on the 2610 do not come preloaded (the 2620 version does, but that has a hard drive rather than flash memory, and can be prone to damage over really rough ground). Although it's been discontinued for a couple of years now, you can still find the 2610 model via ebay shops etc.
Based on your circumstances, I think the Nuvi 205 would be a good (up to the minute) option, and comes preloaded with the whole of the USA maps.
ps. Like all touch screen machines, it's best to put a clear film (like those used to protect the screens of digital cameras) on the GPS screen to protect from scratches due to dust etc.
pps. as for mounting the GPS to the bike - ideally use a RAM mount (cradle and arm) - it really is the most cost effective and secure way to do it. The standard manufacturer GPS brackets tend to be designed for car use, and on a bike the unit could become detatched over rough ground. Factor in another $30 or so for the RAM set up - available in a number of outdoor/marine places typically - I know Bass Pro shops sell them.