I just installed Tiger yesterday and choose the "Save the previous system" install (thus, not a regular upgrade). After doing this, you may want to "reclaim" some space used by the "Previous System" folder.
Some files which are kept and are most likely worthless and can be easily erased are the old swap files. In my case, they were using 1GB of drive space. Here's how to get rid of them in the Terminal:
You can probably also delete these without any fears:
$ cd /Previous\ Systems.localized/Previous\ System\ 1/private/var $ rm -rf vm
In my case, I recovered 2.5GB doing all of these -- not too bad!
$ rm -rf /Previous\ Systems.localized/Previous\ System\ 1/Library/Caches $ rm -rf /Previous\ Systems.localized/Previous\ System\ 1/Library/Printers
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