Hi Harald.g,
1) Using the archive bits is a preffered option because you will actually backup only the files the were modified or the files that was just created.
each time you open an existing file and you make a change in it, the archive bit is ticked and the file is added to the backup list.
and everytime you create a new file , the file is automaticly ticked in the archive bit
this way you save valuable space on your backup tape,
2) it depend if you are using one tape for all the backups or you are using a defferent tape ,
usually you really append to keep old backups ,
you overwrite and existing tape when you are using daily back and lets say on sunday , you overwrite the contents of the previous sunday
Hope this helps
Thanks
Jean