You need a graphic card that supports dual monitor, or those old main board which can take old video card (which can plug into a shorter video slot on the old main board) that allows you to use two video cards on same board. The first option is much more convenient.
I got a Dell in my office which has two LCD monitors connected. Quite convenient, especially for programming and web surfing
Some cards like the Nvidia Quadro PCI-E Series that comes with certain Dell desktop models support a dual display directly. You will need a Y-connector cable that plugs into the card video-out port, and you then plug the two monitors to each end of the Y-connector. You then need to configure your Display Properties to utilise the two monitors.
Additionally, it is also possible to added a PCI video card to an empty PCI-slot if you already have a video card in your PCI-E/PCI/AGP etc. port. That means two video cards to drive two monitors. Again, you need to configure your Display Properties to utilise the two cards/monitors.
Or you could have a dual-monitor card like the Quadro, plus another additional PCI video card. This is my set-up.... 2 monitors on the Quadro plus 1 monitor on the additional PCI video card, giving me 3 monitors.