IRIDIA cluster installing an OS on the servers
You should install a Linux distribution on the servers. Debian has proved itself appropriate and the documentation here is based on Debian. So unless you are a very adventureous spirit with too much time on your hands, you should stick to Debian.
Install Debian in a minimal version
You can get a network installation ISO from for instance: http://www.debian.org/devel/debian-installer/
Notice that polyphemus has two NICs while majorana has three. The 100 Mbit NICs connects the servers to the world, so configure those if you do a network install.
Install a new kernel (version 2.6.x SMP) on majorana. A kernel from a binary image is fine.
On majorana install X and GNOME:
apt-get install gdm gmome-desktop xserver-xfree86 x-window-system
Enable the Gigabit Ethernet NICs in:
Copy the files that you backup to majorana:
and add that path to .bashrc so that scripts in that directory can be executed without typing in your life story.
After Debian has been installed move on to install the cluster server daemons.