If you are going to get Mac OS 10 Leopard (10.5 I believe), and have an external drive, and intend on using the Time Machine, there is a caveat.  As Ars Technica reports, Time Machine will detect changes in files or directories, but not changes WITHIN files.  What this means to you is if a file changes size or other attribute, it gets backed up.

Normally this is fine but in the case of a really huge file that changes just a tiny bit, this may mean that you are backing up multiple gigabytes every time Time Machine performs a backup.  Such a file would be a Microsoft Entourage mailbox file, or a virtualization server sandbox file. 

Ars Technica reports there is a way of excluding specific files, so you may want to take some care to do this ahead of time.