XBMC doesn't really need OpenSSH-server to run, but it can be clever to have if you upgrading the kernel and need to recompile the NVIDIA-driver.
To get full performance of XBMC you need to have the graphic-card driver installed.
If you are going to install the NVIDIA-driver, be sure that you have closed GDM before running the script.
You can close GDM by opening a terminal and type:
sudo /etc/init.d/gdm stop
Then you will see a terminal with no GUI.
Log in with your username and password and run the script by typing:
sudo sh XBMC-installer.sh
It will then install XBMC for you.
This script is tested on Ubuntu 9.10 x86, and should work on Ubuntu 9.10 and above.
Good luck!
If you have any problems, contact me here.