John Gruber has exactly the right prescription:

1. Do a complete backup clone to an external FireWire drive.
2. Test that the backup is indeed bootable and up to date.
3. Unplug the backup drive.
4. Boot from the installer DVD and perform a default upgrade.

This is how I've done the last few upgrades, but when my copy of Leopard arrives next week I think I may do a clean install. I haven't done one for many years, and there's probably lots of accumulated sediment... I could do with a spring clean. I'm only really doing this because I'd like to stop running my machine with its disk 98% full! I regularly use OmniDiskSweeper to find and remove major space-hogs - video podcasts that I watched long ago and forgot to delete, for example. And I long ago moved most of my photos and videos to external drives. But now I suspect it's the thousands of smaller files - logs from utilities I tried under 10.3 and such - that make a significant contribution... We'll see... 100GB ain't what it used to be...