When you do the upgrades as you have done you get unexpected results. For this
reason most users prefer to do a fresh installation each time which gives a
clean start. When you go the upgrade route you can get some settings that don't
work and it will cause you problems sooner or later.

Since you tried different monitors then you can probably concentrate of the
graphics card. To troubleshoot your problem, you will likely need a boot disk,
but your boot disk is incompatible with your system by several versions. I am
not sure why you chose to do the progressive upgrades instead of downloading the
latest version 8.04 and installing from it (not to save bandwidth since it
probably took more your way), but the problem is that now you don't have a disk
to boot from and if you need to reinstall you have to start all over again.

This is the route you went: http://www.ubuntu.com/getubuntu/upgrading
Here is how to install Ubuntu 8.04 from scratch should you have a mind to go
that route:
....softpedia.com/.../...untu-8-04-LTS-84314.shtml

I would suggest that you try to recover by booting into the recovery console and
failing that try to get a proper live CD.
https://wiki.ubuntu.com/RecoveryMode

The problem with this is you are left in CLI only (no graphics mode) and that
may send terror through you. If not, then you can possibly troubleshoot and get
your system up and running.

You can find out the contents of your xorg.conf file from the command line and
edit it or you can try to copy an old version over it. You have little to lose
in either case because your computer can't load X anyway. To see the contents of
your xorg.conf use: sudo nano /etc/x11/xorg.conf at the prompt. If you have some
other editor present such as jed, you can substitute jed for nano. You can then
scroll through the file. Post anything that is helpful to us or you can try to
edit anything that looks suspicious such as the wrong graphics driver. Another
thing to try is to move an older backup into the place of the one that isn't
working like this:
absolutebeginner.wordpress.com/.../restoring-your-previous-xorgcon\
f-file/.

You can also rename or even delete the whole xorg.conf file (sounds more drastic
than it is) and then issue this command: sudo dpkg-reconfigure xserver-xorg

Let us know what you tried and if it worked.