LocalPackages.plist:
The most important one, that you certainly don't want to lose, is LocalPackages.plist. It contains the list 
of the applications already installed on your iPod. If you remove this file, you will not be able to uninstall 
them, and probably not to update them either. It is possible to reconstruct this file manually but it could be 
a long and tedious task if you have many applications. 

PackageSources.plist:
The second one that is worth keeping is PackageSources.plist, it contains the list of the repository sources 
you are currently using on the iPod. If you lose this file, you will have to re-enter the list of your favourite 
sources. 

RemotePackages.plist / TrustedSources.plist:
The two remaining files, RemotePackages.plist and TrustedSources.plist, are reconstructed by the Installer 
so they are not really important. The first one contains the list of the applications fetched from the 
different repositories at the last refresh, removing the file will force a refresh. The second is provided 
by the authors of the Installer, it is updated remotely to give the name of the trusted repositories. If the 
remote version is updated, or if the file is missing, Installer will fetch a new version.