Twin Monitors on Ubuntu 9.04
Nvidia FX500/600 - Dual CPU Athlon 2600
This was such a pain that it seemed worthwhile documenting.

Use NVIDIA; support is not good, but all the others are much, much worse. You must plug in both monitors at install time as they are automatically detected. Both monitors have to be on, including the secondary which will flash like crazy. The motherboard talks to monitors over I2C, so the monitor must be on. You cannot use RGB inputs for the same reason - no I2C.
Synaptic should find the NVIDIA driver and automatically download it, you will probably be prompted for password. Set up the two monitors as Twinview using the NVIDIA facility.
If you do this right, the good news is that you will have two monitors working as extended desktop. The bad news is that the NVIDIA facility will not be able to save it.
Hit the preview button to see the new xorg.conf file, select all, copy and paste into a text editor in the vain hope that you may understand what the hell it is doing. Open terminal and start with sudo -s. Navigate to root/etc/X11/ and copy xorg.conf to xorg.conf-backup just to be safe. Then type: gedit xorg.conf and paste your new xorg.conf from the clipboard. Save xorg.conf and reboot.