Multiple Monitors

Even simpler test!

Just swap one of your monitors to the onboard VGA output and see if it's displaying!

No

What motherboard do you have?

According to Belarc=> Board: ASUSTeK Computer INC. V-M2NC61P 

A google search shows this is an 'V-Series M2NC61P Asus PC (Desktop Barebone)' and reading the manual shows (in the BIOS/Advanced/Southbridge Configuration)

*Primary Graphics Adaptor [PCIE -> PCI -> IGP]*
Allows you to set the display decives priority.
Configuration options: [PCIE -> PCI -> IGP] [IGP -> PCI -> PCIE]

Usually the motherboards which disable onboard video actually do it on a hardware level - you would not see a display adaptor listed in device manager, this would be your next port of call to see if the onboard Graphics Adaptor is enabled with either of the 2 BIOS settings, if it is then you might be able to use that setting (aslong as your 8400 works) to have 3 displays...

If it doesn't your next step would be a 2nd Graphics card as explained by josh3736 below, but just remember - your motherboard only has ONE PCI-e x16 port (and 2 'standard' PCI).

Simply unplug one of the monitors from your graphics card and plug it into the main board.

If you get a display you know that you can buy a new monitor.

Since you commented that after connecting one monitor to the IGP and one to your graphics card you only had the one attached to the IGP working I think you're out of luck unless you get a card that supports 3 monitors natively. Any ATI5xxx 6xxx or 7xxx card will do this provided at least one of your monitors supports displayport. If you're buying a new monitor this isn't hard to get as long as you're not determined to buy the absolute cheapest monitor you can find. You can connect a non-DP monitor to a DP port on an ATI card; but to get 3 monitor support you'll need to use an active adapter; these are on the chunky side and run about $30. The much smaller/cheaper passive adapters won't let you add a 3rd monitor.

Most ATI/nVidia cards will only support a total of 2 monitors across VGA, DVI, and HDMI. There are a handful of exceptions; but they're the result of customization by the manufacturer so you'll need to read the specsheet/box to find them; it's not as simple as "any XYZ series card will work".

You need to check the bios. There may be an option in there for enabling/disabling the on board video.