If you have lots of music you want to carry around with you, then maybe a 64GB is useful to you. How much space does your current music collection take in iTunes now?
Or of you like to carry lots of pictures with you, or you like to carry lots of videos (either movies, TV shows, or video podcasts), then maybe 64GB would be useful.
If it is just apps, then they really do not take up a lot of space, so a 32GB is generally more than enough. I have 32GB with a generous subset of my music, a movie or two, a few video podcasts, several hundred pictures, and about 30 or 40 apps, and I still have about 10GB free.
Then again, I have a friend that keeps hoping for a 128GB iPod Touch so he can put his total music collection on it. He is going to have to wait a while longer for something that big (assuming the maximum size eventually gets larger 🙂 ).