"Loss of Data" describes a pretty huge range of possibilities.
Exactly what kind of data was lost? Applications? Prefs? Files? Documents? etc.? Did you do the upgrade via online Software Update pane or download it from apple.com?
Have you done a search on your drive(s) for the missing data, in case it somehow got moved from where you expect it to be?
I did the 10.4.11 update many months ago on a G4 MDD with no apparent problems.
BTW, Get More RAM! 512MB barely enough to support the OS in addition to memory needs of applications.