Thanks for your help, Toucan!
Your 3rd link isn't working:
My server is an co-location server, at an ISP broadband broadband facility. No firewall. I currently run ISPConfig 2 on the server. For testing ISPConfig 3, I just shut down i2 and bring i3 online instead, using the same IP. My i2 works fine, hosting i3 testing websites with their own domain names. I fired up i3 and entered DNS settings that should resemble something similar to the settings I have on i2. I use Ubuntu 8.04 as a host on both.
Is i3 worth the effort or should I stick to i2? I'll guess the development is focused on i3. That's the reason I want to do the switch.