In that case the process will be "non-trivial" :)
I'd suggest the following procedure:
Take a full copy of the drive while the machine is
off-line (e.g. using a boot-cd) so you could just
swap the HDD in case of a physcial failure.
As for the ongoing work:
You'll have to set-up two jobs (possibly both
triggered by the same cron-event). One will be
to mysqldump the database and then copy that
file to the other drive (you may want to consider
version-keeping, like a weekly roll-over, using
e.g. the rsync backup-script). The other should
be an rsync of your www-directory which should
be no big deal at any given time.