As for 8GB vs 16GB, I have a first-gen 8GB and have found I use it for a lot more video than I would have thought I would. As a result, I tend to have ~4GB music, some music videos, a few gigs for video (podcasts,TV,movies), and a small amount left over for photos and "other". I'm always close to full, yet don't hit the limit that often. As long as you don't feel the need to have the entire series of whatever you are watching, but instead just have a few episodes, and generally only 1 movie at a time at most (mine tend to range ~700MB-1.3GB), you can stick under 8GB fine.
I control my music via smart playlists, so if I decide I need extra space for podcasts/videos etc I can reduce my music allocation very easily by just reducing the max size of one playlist, and this removes some recently played songs, yet leaves all my favourite songs still on the phone (they have a separate playlist). Even if you have selected a specific playlist for your phone (or have your entire library if it is small enough), I recommend wrapping it in a smart playlist like this, as it is very convenient for being able to temporarily reduce your music without actually changing the underlying playlist.
Jailbreak apps are pretty damn small (most are <1MB), and I presume the new apps will be similar, though there may be some cool App Store app that requires lots of space to store stuff, who knows.
I control my music via smart playlists, so if I decide I need extra space for podcasts/videos etc I can reduce my music allocation very easily by just reducing the max size of one playlist, and this removes some recently played songs, yet leaves all my favourite songs still on the phone (they have a separate playlist). Even if you have selected a specific playlist for your phone (or have your entire library if it is small enough), I recommend wrapping it in a smart playlist like this, as it is very convenient for being able to temporarily reduce your music without actually changing the underlying playlist.
Jailbreak apps are pretty damn small (most are <1MB), and I presume the new apps will be similar, though there may be some cool App Store app that requires lots of space to store stuff, who knows.