theres probably a few ways. Ive chosen to sync using rsync over ssh from a cron job on a desktop pc running ubuntu. For this to work, you need openssh and rsync installed on the n900.
Alternatively, you could have a play with "GUI frontend for rsync" in app manager (make sure maemo-extras is enabled). This would require a rsync server installed on your PC. Theres windows versions of rsync available if its running windows. Have a look at cwrsync.