i thought that i would have enough room with a 30gb hard drive and 2 external hard drives. i bought a used ibook that came with a 60gb hard drive and i have since filled that as well as both of my external hard drives. the os and applications took up more room then i was originally thinking so i am happy that i have the 60gb.
I would say it depends on how much music you'll put on there. If you get a 20g or 40g iPod, you're going to load up your iBook with a lot of music. Same goes for movies, pictures, etc.
I have a 40g iBook. It has 16g of music, plus regular stuff including MS Office and I have about 8g of free space. That's after I have removed Garageband, Mail, and Calendar, not to mention some other smaller apps. I'm soon ordering a 100g HD Powerbook since the iBooks didn't come out with higher res screens.
I was thinking about this in the morning, and I think it will barely make it.
Keep in mind that the real size of these drives is 37GB or something.
My home directory is over 15GB with the music, photos, and other docs that I have. That leaves me with around 12 GB on my "30"GB HD. 1-2 GB for virtual memory and I am down to 10 GB for the system, library, and all applications.
I had to move some applications off to an external drive, and I used to keep disk images for games on the HD to be able to play without the CD. I can't do that anymore.
I think 60GB should be a minimum, go for the max if you are doing any iMovie work.
I have a 60 GB HD in my powerbook, and I was tempted to upgrade to a 100 GB HD when my original failed. I didn't, and I realize I only use about 30-40 GB of HD space.
However I do have all my music on my iPod, so that may make a difference depending on your music collection. Otherwise for everything else I burned onto DVDs ... I have approximately 200+ GB burned onto DVDs, and while some may think it's foolish, it works fine for me.
It all depends on what you intend to keep on the HD ... for light libraries of music (5-10 GB), pictures (5 GB), Applications (10-15 GB) you should be fine with 40 GB including other stuff. Just leave some room to expand into ^^
I'm on a 60 GB HD on my current iBook, and with a ~35 GB iTunes library that's just way too small... I'd gone with a 100 GB if I ordered today (just as 60 was max when I ordered my iBook G4@800) or, maybe, the 80 GB if I was on a strict budget (which I really hope I'm not whenever my 'Book needs replacing).
Take the 100 GB... you'll run out of space soon enough...
As always, depends on what you'd do with it. If you mean just the OS and apps, it will be decent. Not great, since things like GarageBand can take up GBs just by itself, but it will suffice. If you do video/audio/imaging, it will fill up fast.