If you're still using the original stock card, then correct - one monitor per card only.
If you're using a newer card, then most likely you can have two displays.
I have a GeForce 4 Ti 4600 in mine, and it happily runs both a 17" ADC Display (1280x1024) and a 21" ViewSonic 1920x1080 DVI display at the same time.
I would not recommend adding a second PCI video card, but rather upgrading to a new dual-monitor-capable AGP card, such as this one: http://www.ebay.com/itm/170594386309
If you're using a newer card, then most likely you can have two displays.
I have a GeForce 4 Ti 4600 in mine, and it happily runs both a 17" ADC Display (1280x1024) and a 21" ViewSonic 1920x1080 DVI display at the same time.
I would not recommend adding a second PCI video card, but rather upgrading to a new dual-monitor-capable AGP card, such as this one: http://www.ebay.com/itm/170594386309