It actually is hard to say.
I think a 2 hour movie may take up about 600mb of space and 200 songs (assuming 4mb a song) would be about 800mb. For apps, it is hard to say, if they are productivity apps, they can be a few mb each but games can go up to a few hundred mb each, some I see are over 500mb in size.
No matter which size you get, you would never get the full advertised capacity. I believe a 8gb iPod Touch 4, you will have something close to 7gb of free space to work with. You never know with future iOS updates and new features, it would inevitably eat in to the space.
In my opinion, the 32gb is a good choice as the 16gb option is no longer available.
In my 32gb iPod Touch 4, I have a capacity of 29.1gb space. With 2580 songs, 6 x 30minutes iPod U cast and 122 apps, I have about 10.8gb free space now.