Update Nov. 9, 2008 - While trying to get Ubuntu 8.10 working on my 2600, I remembered one key detail to getting 8.04 to work. I've only gotten it to work with BIOS revision A08. Here's a link to the Inspiron 2600 BIOS A08 on the Dell website. Unfortunately, this is a Windows executable to make a bootable floppy, so obviously you'll need access to a Windows machine to make the BIOS boot disk.
Ubuntu 8.04 installation on the Dell Inspiron 2600 takes a few tweaks to the video configuration to get it working properly. First off, to get the Live CD to work you will need to select Safe Graphics Mode at the boot options screen to get the Desktop Environment to load properly. I'm not going to cover the entire Ubuntu installation process as it is well covered elsewhere.
Once you have Ubuntu installed you need to reboot and select the recovery mode from the boot screen. Select the root prompt option and hit Enter. Now that you're logged in, you need to edit the xorg.conf file.
Find the section with the following line:
and change the section to look like this.
Identifier "Intel i830"
The next section should be for "Configured Monitor". Change it to look like this:
Identifier "Generic Monitor"
Next, change the "Screen" section to look like this:
Identifier "Default Screen"
Device "Intel i830"
Monitor "Generic Monitor"
Modes "1024x768" "800x600"
With all of your edits complete, hit Ctrl+X to exit the nano editor. Hit Y when prompted to save, then hit Enter to save it as /etc/X11/xorg.conf.
Now, reboot your computer to see the changes take effect.