To install the packages on Ubuntu 8.04
you need to
sudo vim /etc/apt/sources.list
and add this line at the end
deb http://debian.adenu.ia.uned.es/apt hardy
Save the file then do
sudo apt-get update
From there you should be able to install all the dependencies for OpenACS with this command
apt-get install aolserver4 aolserver4-nscache aolserver4-nssha1 aolserver4-nspostgres postgresql
You probably still want to download the latest release of OpenACS or get it from CVS to complete the install which means you'll need to edit the config file to point to the right places. Most of the default config file should work on an Ubuntu install.