This is a general hint that many of us could figure out, but probably is overlooked by many people, particularly if you've had your Mac a couple of years.
Software update likes to insist that you have and download the latest version of iPod Updater and iSight Updater - even if you aren't using these peripherals (at least on my Mac). It nags you to obtain these updates, though you may not need them, by leaving them on the checklist of files.
Unfortunately, it doesn't delete the PREVIOUS updaters that were downloaded. I found, by accident, that these were eating up a lot of space. Lots!
My 12" G4 PB is about 2 years old, and I've downloaded every iPod updater and iSight updater since 2004 May. Well, my 60Gb hard drive is darn near full, so I've been looking for ways to clear up space - including deleting extraneous photos and music I don't need.
Well, I was looking through my App folder and saw all the iPod updaters together. For kicks, I did a GET INFO to see how much HD space was being used by the folder. I was shocked to find it was well over 600MB!!
I deleted all but the most recent version of iPod updater in that folder, and the same for the iSight Updater, and now my HD is much more comfier.
Let's hope we can get Apple to fix this glitch in the way Software update moves/assesses the updates for these peripherals.