easiest way would be is to get a dual monitor card

assimilate all tech.

Please keep me posted on your progress, as I use multiple monitors,
but in Windoze OS and am quite interested. Most of my presentation
laptops for video projectors are equipped, but quite a hassle in MS.

Don;t know how to verify their existence in ubuntulinux, would like to know.
If you have MS installed, you might first verify your present hardware.
Many times they contain second monitor capability just lurking.
It's the ol' "Start / Control Panel / Display / Settings" trail.

I have a dozen computers, some of which had second monitor
capability of which I had not been aware.

At least a first step.

I shall look for another 'dual' monitor card.

Could I simply add a second Graphics card PCI type as an alternative?

yes you could as long as the system recognises it try and get the same as what
you've got

I'd vote for a replacement dual-head graphics card too but being AGP
your choices are beginning to get limited for new ones. Might need to
go for EBay but check the vendor is good first.

Once you have a dual-head card installed then plugging in the second
monitor will automatically give it an entry in display management
where you can set the parameters such as screen size and whether it's
a separate screen or extension of the main one etc.

I'll begin to search for another card.