Can you replace the address field and the address book button by a pull-down field which combines the functionality?
Yes good point -- I guess a combobox could be used there.
Is the add to address book line greyed out, when one uses an address from the address book? I suggest that the checkbox is deactivated by default, and automatically activates, when the user enters anything in the name field.
The 'add to address book as' checkbox is disabled by default, and you can only enter a name when it is enabled (otherwise the input field is disabled and the tab focus skips directly to 'amount'). I think this is more clear than the other way around.