in response to darren's new mac, I thought I'd post some things to get him started, and no doubt others will add their own comments.
first up, for multimedia authoring, I have my hard disk partitioned. why? so that I have a boot-software disk and a 'project' disk. it means that I can clear and defragment the latter without the tedium of having to defragment the former.
you can partition the disk on setup, but if it's already set up, the tool to use is iPartition, with also comes with iDefreg Lite. iPartition lets you change the size of your disk partitions later, which is brilliant.
I think a 40 Gb project disk (minimum is necessary). Mind you, more and more Apple software seems to want to be on the boot drive. In some cases (such as iPhoto and iTunes), you can simply move the folder and put an alias where its supposed to be (in the Pictures or Music folder in your user folder).
And when you format the second partition, make sure that journalling is turned off. (use Disk Utility to erase and reformat it if you like).
Some folk suggest making the project drive the first partition, as the computer will access it quicker. I think that's mainly an issue for multitrack audio recording, when you really do need maximum disk access speed.
The second initial thing was to install the XCode Developer tools - you'll want some of them later.
The third thing is NOT to install all of the additional languages or printer drivers - they take up bucketloads of hard disk space.
More coming soon - Quicktime stuff, essential free and cheap software...
BTW, I usually let Software Update tell me what is needed. and then go to the Support section of the Apple website to download the required files. Why? So I can burn a backup copy of the file in case I ever need to reinstall it and don't want to have to download gigabytes of files again.
(I use DiskTracker to catalog my backup disks - it doesnt look fancy but it does the job.)