You manage all your songs via iTunes. You can "uncheck" songs that you don't want to go onto the nano, while leaving them on the iTunes, in case you want them later. You can also create "playlists" such as "workout", "cooldown", etc. that will play selected songs. Play with it a bit. It's pretty managable and easy to work with (like all Apple products) once you get the hang of it.
My house is now a 15g iPod/mini/nano/shuffle household. We're iPod freaks. And it's only my wife and I.
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Father Kevin
http://www.churchofcycling.org