Depending on the age, a lot of the small form factor business PCs are already dual monitor capable, usually having a DVI port (which may support two monitors as-is), or a VGA port, and another port like a displayport, mini displayport or hdmi.

My last two cheap spare win7 boxes have been hp/compaq business PC lease returns and supported dual monitors from the onboard video via a vga connector and displayport connector, and a dvi and hdmi connector, respectively.

you'll likely need dvi to vga and other such converters to homogenize your connectors, depending on what inputs your monitors can accept.