This is what caused my problems from earlier when I tried to move to 9.04 beta release.
How you get it running on this system is as follows.
Open up your case and pull the VGA output off the graphics card, this tells the card that it only has one output.
Start up your machine and open the BIOS settings and tell your machine to use the pci-express graphics card before the on-board one. Dell machines normally complain otherwise.
Install [K]ubuntu 9.04 as normal in whatever layout you want.
After installation, get your system updates and enable the restricted nVidia Drivers (V. 180)
Reboot and check your machine still works and that you now have the nVidia Server Settings option in your K menu (or wherever it goes for Gnome)
Power off after testing and plug your VGA output back in on the card, attach your second monitor
Open up your nVidia Server Settings application and enable the second monitor. DO NOT CLOSE THIS APPLICATION YET
Open up a terminal window (ALT+F2 "konsole") and enter
sudo chmod a+w /etc/X11
Back in your nVidia Server Settings app, hit "save to X configuration" and you are now done. Dual screens persistent across reboots.
TA bloody DA.