This howto is written for the old Joomla.

I was able to get the new Joomla 1.5 working, using the howtoforge howto with ISPConfig3 on CentOS 5.2:

http://www.howtoforge.com/perfect-server-centos-5.2-ispconfig-3

After you build the CentOS/ISPConfig box, and make sure everything is working (yes, that howto works), you can install Joomla 1.5 on it.

 Download the new Joomla archive file, put it on your website.  I created a directory called 'installation', and unpacked the Joomla archive file in there.

Before you run the installation routine, you have to edit your /etc/php.ini file, and make some changes.

vi /etc/php.ini

register_globals = Off

memory_limit = 128M

session.save_path = "/tmp/"

That's it.  Find the above lines, and change to look like what I listed here.  Reboot the server (probably not necessary, but what the heck), connect your browser to your web site's installation directory, and follow the prompts!  You should be good from there.