It is possible to run more than one monitor at a time (different cards or with a card with dual outputs) under Windows 98 and up. However, as jr76 said, you can't really have two people use the system at the same time, do to input issues.
Running dual cards is simple, just install the PCI video card and Windows will recognise it. Go into display properties, settings, click on the box labled '2' and click the checkbox down below saying 'Extend Windows Desktop Here'. Your second monitor flickers and comes up with Windows (usually with no icons nor taskbar) and you can drag windows from one screen to the other. Most full screen stuff defaults to using the first display (should be your ArcadeVGA by default), no matter where you launch it.
Running multiple monitors does slow down your system though. OpenGL based games usually kill the second display, Direct-X games usually leave it running (though you can't interact with the second screen).
Heres a screens shot of me running MAME on monitor 1 (PC monitor on right) and Winamp on monitor 2 (TV on left):

As you can see, they can even be running at different resolutions. You can drag around monitor in the settings tab to tell Windows where it is in relation to the first monitor.