We have a guy running VW 11.5.1 on an iBook ~900MHz it runs very slow for him, other people have newer iBooks/PowerBooks and it runs fine (the new iBooks are much faster then the old ones).
Coming from a dual 867 G4 you shouldn't notice too much of a performance drop in fact you may see a performance gain (depending what OS you are using now).
Personally, I'd love a 15" PowerBook, but if it were me I'd be buying the 14" iBook the CPU difference is not that great any more to be too much of a factor, the only thing you have to look at is the difference in video card's (I am not sure how much of a factor that is going to be with VectorWroks).
Also, with the money you save getting the iBook you can have it come with ~1GB of RAM.