Muliptle Video Cards in Optiplex 960

I currently have a Dell Optiplex 960 with an ATI Radeon 4670 with dual monitor support.  It currently plugs into the PCIe x16 port on the motherboard.  I am wanting to add one more monitor here but here is my problem.  I tried just adding a third monitor by either plugging in the monitor into the on board analog vga output and got an error message that this configuration was not supported.  I also tried going through the DisplayPort as well and received the same error.  Apparently whenever you are using a card in the PCIe slot, it disables the on board video.  I then found a video card that I plugged into one of the other pci slots and plugged in the monitor to that.  I booted the computer and set the BIOS to auto.  Now what happens is that the third monitor that I have hooked up is the only monitor that is operating and the other two monitors that are being driving by the ATI card won't function.  If I go into the BIOS and set it up for "on board", then it disables the third monitor and the original two monitors will function properly.  How can I get this computer to utilize both video cards, one being in the PCIe slot and the other being in just a regular PCI slot.  This is very frustrating because I have done this with other computers in the past and have never had this issue.  The funny thing about this whole thing is, if I leave it where only the third monitor is functioning, you can go into device manager in Windows and see both cards listed and when going into the properties, it says the device is working properly, but when going into the display settings where you would normally setup your displays, it only shows one display instead of three.  I am running Windows 7 64 bit also if that helps.

Thanks for your assistance.