make sure you get one with a VGA input.
You will be limited to mirroring the 1024x768 resolution of the iBook's internal monitor, so you won't actually gain any real estate.
Unless, of course, you hack the chip using one of the freely available iBook screen enhancer utilities (since the chip is capable of supporting *additional* monitor space - called "screen spanning", at least theoretically).
Note, however, that the chips Apple buys cheap in bulk for the iBook are NOT tested for that particular function, and that the cooling of the chip is not designed for the additional load.
So this *may* work for a short while, or even a long while, but you *do* risk frying your graphics chip, which can only be replaced along with the entire main logic board ($$$), and, should the hack be identifiable in the machine's firmware, the repair will NOT be covered under warranty or AppleCare.
If you really need an additional monitor or larger screen real estate than 1024x768, I would *not* recommend doing this with the iBook.