Even simpler test!
Just swap one of your monitors to the onboard VGA output and see if it's displaying!
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).