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.