For example, I have several full-length operas on my iPod but I don't like some random aria popping up when I shuffle my music. So I just create a Smart Playlist that blocks all classical music. I also have a Smart Playlist that plays nothing but songs with 4 or 5 stars in the Rock genre and a Smart Playlist that only plays pop music from the years 1974-1979.
The combinations are endless.
Now I'm loading all the album cover art into iTunes so when I upgrade to the 60GB photo, I'll be ready.
Classical music and opera are different beasts anyway. I could never get into hearing different parts of different musical works jumbled together in one shuffle.
I'll have to play with the smart playlists some more to see if I can accomplish what you have.