Step 1) Setting up openSUSE 11.0

I won't go into great detail on installing as the installer should get you through the steps easy enough.

Basically stick to the install defaults and I would recommend using the install DVD so you have all the needed packages at hand. Get that here : http://software.opensuse.org

Note on DE choice: I use GNOME as my window manager, but this is not really relevant as this is a monitoring system and it's cleaner to run it in console mode to keep a small footprint.

openSUSE Install guidelines & considerations:
(if you are comfortable with setting up or already have a running system, skip to step 2)

Partitioning:

swap 2 GB (swap)
/ 15 GB or more (Reiserfs)
/home 3 GB or more (ext3)

For your root partition keep a minimum of 15 GB to ensure there is enough space for logging etc.
The home partition is optional and can be used to keep a copy of you configurations and install files in case the root partition gets damaged.

Create a system user to work with (avoid using root as working account). In this example I'll create a user 'system' with password 'manager'. Do note that Nagios configuration files might contain info considered as sensitive. Keep this im mind when creating the accounts.

Network services:
Firewall service is not needed but if keeping the firewall service enabled, open the port for ssh.
I would recommend to NOT enable Remote Administration (VNC) as ssh gives you all the managing power needed with Nagios.

Configure a static ip address for your Nagios system to ensure you don't run into a failing monitoring system due to DHCP issues.

Keeping the correct time:
Setup the Nagios system to sync it's time using NTP. Logging won't do you much good if the reference time is off course.

Apply all openSUSE updates:
After installing openSUSE and before configuring Nagios make sure all system updates (using the Online update tool in YaST or from the command line) have been applied.
To update from the console command line run:

zypper up

* always reboot after kernel updates have been applied

Running a lean system:

Disable openSUSE updater applet (Control Center > sessions) or remove the start item:

rm /usr/share/gnome/autostart/gpk-update-icon.desktop

Beagle indexing is unneeded in this setup, to remove it:

zypper remove beagle

(Optional) Turn off the powerdeamon:

chkconfig powerd off