I am running a Dell Dimension 9200, Core 2 Duo 6600 2.4 with 4GB RAM. I have tried two different video cards that support dual display, and with both only one monitor is seen in display properties and also in the Nvidia software. I used Nvidia's latest greatest drivers in both cases, and reverted back to the Windows drivers each time to clear everything out.
The first was a 7300, and the one I really want to work is an MSI Nvidia GeForce 8600GT. Both work fine, but there is no second display available. I have confirmed that both monitors are working, they are Dell 2407WFP's.
Attached are screenshots of my Nvidia CP, and Windows display CP.