The Nuvi or for that fact most of these types of mobile GPS units are quite easy to incorporate into Geocaching.
There is an excellent step by step tutorial by Pilotsnipes. Here is the link:
http://pilotsnipes.googlepages.com/index.html
It basically tells you everything you have to know to get caches onto the portable device.
There is another more complex one that does several things differently, but the one above is the best place to start.