You sound like a perfect candidate for the iBook -- the real differences between the iBook and PowerBook are currently
1. Maximum RAM support (iBook: 640MB; PB12": 1.25GB) -- doesn't sound like too much of an issue for you, especially if you get an aftermarket 512MB DIMM (approx. $140)
2. Superdrive (iBook: not an option; PB12": optional) -- only you know if you are going to want to burn a movie dvd; if so, you might want to spring for the PB since you get not only the Superdrive, but also iDVD (saving you big $$ over DVD Studio Pro)
3. FireWire800 support (iBook: no; PB12": yes) -- I haven't seen any test where FW800 actually bechmarked at 2x the speed of FW400, and your projected uses really don't sound like this should be a concern
4. DVI support (iBook: VGA only; PB12": mini-DVI) -- as I understand it, there's no magic to DVI over VGA, but a good VGA image on an external monitor requires the monitor to have a good digital-to-analog algorithm. Unless you foresee this being a problem, it shouldn't be your choice criterion here.
So, from what you've described as your prospective use, I'd objectively recommend the iBook. HOWEVER, if you can afford the PB and WANT IT because:
a. It has the word "Power" in its name
b. It's made of metal
c. It's got a metal keyboard
d. My friends have iBooks and this is a chance to one-up them
e. My friends have iBooks and this is my chance not to be one-upped
f. I won't use [FW800 / SuperDrive / DVI / iDVD / 1.25GB RAM / etc.] but it would be cool to be able to.
g. I know I won't be burning my own DVD movies, but I like to think that I might and the PB will let me hang on to that fantasy a while longer
h. [other reason that people don't fess up to]
THEN -- I say BUY the 12" PowerBook.
If you can afford it and it makes you happy, then that's what this is aaaaaall about! Get the Mac that you'll enjoy.
Best of Luck!