You could also try Disable Tiger Features:
http://rixstep.com/2/20050528,00.html
In addition to Dashboard, it allows you to disable Spotlight.
While this may not directly relate to the OS upgrade, when I got my new computers, they came with Tiger and there were a bunch of new apps on them that I did not have before (or are now much bigger). One of these is GarageBand. I never use this app (though it looks fun). The other day I was surprised to see that the Instrument Library for Garageband occupies 1.35 GB! Another 1.1 GB of "Apple Loops for Garageband" Another space hog: Printer profiles at 1.64 GB on my machine. (all of these are in the main Library folder) Hidden away in Users/Shared are GarageBand Demo Songs, but that's only 367 MB. Some others: iDVD: 1.46 GB, iWork: 542 MB.
OK, on a tower with a 250+ GB drive, that's not a big deal, but on my laptop, when I'm regularly stuffing it's 100 GB drive with thousands of raw files... 6+ GB can certainly be useful. I just wish I could convince myself to delete most of the printer profiles.. but the funny thing is with a laptop, you never know when you'll be at someone's office and need to print something.