(Since commenting is not available, I add another reply..)
A word of caution for the advise "You could also image of the HDD (with ESXi offline), and so you can keep a second disk with your setup, for quick recovery." in the first message. Many people are having issues with cloned or imaged ESXi 4 installations. Usually what happens is what user blambert describes at virtualizationtimes:
I have ESXi running on a single drive
w/ VMFS partition. An image of this
drive has been created using Symantec
Ghost. Whenever the image is restored
to a new drive, the datastore is no
I can manually remap the VMFS
partition using add storage.
If I add storage and do not
resignature the drive, the datastore
is no longer mapped after rebooting.
If I add storage and do resignature
the drive, the datastore remains
mapped after rebooting, but any VM's
show up as inaccessible and must be
readded to inventory after each
Imaging/cloning ESXi is not officially supported, so it's a safer route to back up the settings the 'official' way with vicfg-cfgbackup like Chopper3 suggested.