While looking over other’s comments, it occurred to me that unless you are listening to one shiur after another, without interuption the entire day, and will not be able to get near your computer to sync your Ipod on a regular basis, you can easily get by even with an 8G Nano. Let’s assume you’re listening to shiurim (larger files then music). If you sync your Ipod every so often, you can easily change what’s on there, since all your files remain on your computer in your Itunes library.
For example, in addition to having shiurim of all types on my Ipod, I like to make playlists by themes, such as Pesach, Shavous, etc. When Pesach passed, I unchecked all my Pesach shiurim, and removed them from my device (you can set up the Itunes to only copy checked items).