>Now my question: When I want to make a Begin Transaction - End Transacation, do I have to Begin the transaction before I make the changes to the buffer, or just before I do the Tableupdate ?

Before the TableUpdate(). The transaction can be rolled back if something bad happens. Putting data in the buffer is already revertable, so when you commit you want the ablility to rollback too, and thats what the transaction is for.