Onboard graphics usually have a VGA output (depending on the age of the computer). Some pc's already have either DVI or HDMI.
Running two monitors I would suggest getting a Video card with two outputs (preferrable same connectors), then you can control each monitor through the video card. However, I would not use PCI as it is sometimes slow (use PCIe if possible). Depending on the age of your pc you might be able to use AGP with a GeForce 5200 (what I am using). Never used the onboard video card.
Since I do not know your hardware, I can only give general advice