I have a Dell 8300 with a dual monitor AGP card. Can I add a dual PCI video card for a 3rd or 4th monitor?

unfortunatley this will conflict and you'll need to turn one of the video cards off

look for a multihead video card

they are rare but did support 4 monitors

or buy a video split box i believe the company name was kvr

kwr something like that

they make multiswitches and they have one that will hookup several monitors at the same time for mega desktop

just not 4 separate desktops