I just reinstalled my Cruncher, and it's woking fine. So I developed this guide to help in a full installation with Ubuntu 11.10. I hope this guide can be of help to expert and non-expert Linux users.

I will put on green every shell command.

First I recommend you to unplug all the NVIDIA cards, If you don't you might get a black screen as It happened to me. Maybe you don't have that problem, but I wouldn't risk. You will read everywhere how wonderfull Linux is, but nobody talks about how frustrating can be when you have hardware problems.

Installation of Ubuntu 11.10
Go to Download Ubuntu
Choose 64 bit
Download (ubuntu-11.10-desktop-amd64). 64 bits needed for GPUGrid!
Burn your CD or create a bootable USB stick
Try it or Install it directly

Upgrade
Instructions to update are here
Instead of using this program you can do the same with the shell:
sudo apt-get update
sudo apt-get upgrade

Download Drivers
Download latest nvidia drivers
(don't forget to write down the path where you download them)

Blacklist video drivers
You can do this later in the shell, but it's easier do it with gedit, so I recommend doing it now.
sudo gedit /etc/modprobe.d/blacklist.conf
Add these lines at the end of the file and save:
blacklist vga16fb
blacklist nouveau
blacklist rivafb
blacklist nvidiafb
blacklist rivatv

Recovery mode as root
Plug in your card
Restart your computer.
When it startsup leave 'SHIFT' key pressed.
You will see the GRUB loader like this
Over the 'recovery mode' line press 'e'
Chage 'ro recovery' to 'rw recovery'
Remove 'vga=XXX splash'
Press 'F10'
Now you shoud see this screen
Start in root mode

Installation of NVIDIA drivers
As you are in read and write superuser recovery mode 'rw' you don't have to use the sudo command on the following lines:
Uninstall nouveau
apt-get --purge remove xserver-xorg-video
Uninstall the older drivers
apt-get --purge remove nvidia-*
Goto the path you downloaded the NVIDIA drivers
cd xxxxxxxx
Install NVIDIA drivers
sh NVIDIA....pkg.run
Answer yes to all questions from the installer, but be really carefull to read any advice

BOINC instalation
(you still at root, but you can do this once you have restarted your computer)
apt-get install boinc-client boinc-manager
64 Bit Considerations
To allow 32 bit applications to run on 64 bit Linux you must install 32 bit versions of certain shared libraries.
apt-get install ia32-libs libstdc++6 libstdc++5 freeglut3

Now you can restart and configure BOINC and Ubuntu as you wish.
If BOINC don't recognize your GPU card put a delay in the start up

More info about GRUB
NvidiaTroubleshooting