Forget all this other junk. Your needs are so simple that you're thinking about this too hard. You're also not going to be using it for a long time.
Any iBook will do what you want, even if its a 12" iBook G3 800 MHz or something. If you want a 12" G4 iBook, get an 800 MHz, as it's probably more tolerable. All this stuff about superdrives, Core Image, screen size, etc...... isn't an issue. Unless you have poor sight, you'll be able to see things on a 12" screen just fine. The text on the 12" screen may actually be larger than it is on a 15" PB because of the difference in resolution, and people seem to be fine using either machine, so don't worry.
There's no such thing as a 12" 667 MHz PowerBook. The slowest 12" PB ever was an 867 MHz G4.
Get an iBook over an older PB for your needs because the iBook has better battery life and better wireless reception. Don't worry about CPU speed as long as it's a G4 and faster than 800MHz. If you can get an 800 MHz iBook G4, then get it. Otherwise, a G3 iBook isn't horrible for what you want to do.