Hi David, I'm pretty sure it has to be a 9.5mm tall hard drive. Some laptop hard drives are 12mm tall and these will not fit. If I were you I would go for the largest hard drive I could afford, as even 20Gb can fill up pretty quick once you own those 20Gb. The 320Mb limit can be upgraded by upgrading the firmware which makes the iBook recognise more than 256Mb in the memory bay thing. Apple has this available as a free download from their website and its only a couple of megabytes to download, then you're all set for juicy 512Mb module goodness. The very first clamshell iBooks (i.e. The 300MHz Tangerine and Blueberry 1999 models) can also have the 512Mb module installed.