Coming from 32GB N900 - I had never gone to more than 10GB used. I have never deleted a photo, having like ~1400 on it.I am not not much of music fan though so there was less than 1 GB on it, just some hard-rock classics
Now I own 64GB N9 which looks like overkill to me, but I have gone for the white color. Looking from another angle to it, I am happy that I have this 64GB now 'cause otherwise I would have always had doubts "I should have spent the money and bought 64GB white one!"
My advice to @user_nameless - see if you can easily sell the one you got now for a day or two, then get 64GB. If you can not sell it so fast on a reasonable price - just open it and enjoy!