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As long as my library was small enough to fit on a 30GB iPod, it made sense to me to keep the full library on the iPod.

Once the library grew past that point and I had to limit the items to sync I realized that with a set of smart playlists I could probably get by with 8GB and have 99% of everything I'd normally want to listen to on the go synced. 16GB would add room for a couple movies/TV-series too.

Everything is completely and to my knowledge correctly tagged, so navigating, finding stuff etc. vs junk isn't really an issue for me.
 
Like the second poster said....if you organize your music there is no reason you cant find what you are looking for. I am meticulous about making sure I have the album, cover art, name of song, and artist all correct even if I download it from somewhere other than itunes.
 
ideally, 200+GB. realistically, my current 16GB would suffice. I have no problem replacing songs and videos, but would love the fact that I don't have to worry about storage all the time. I still have about 5GB left on my 16GB iPhone, but that can be filled in seconds.
 
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