Cant enable third video card (PCI) - P5QL Pro

I am running an EVGA GeForce 9800GT PCIe with two EVGA GeForce 8400GS PCI cards on an ASUS P5QL Pro Mobo. I need to be able to use 5 or 6 screens but the computer will only detect the PCIe card and one PCI card (detects only one PCI card at a time, won't detect both simultaneously). This is for an office and I've already purchased the parts (for 5 PCs), so I'm looking for a solution which does not require buying anything else if possible.

PC Specs:
Mobo: Asus P5QL Pro
CPU: C2Q Q9650 3.0GHz
RAM: 4GB DDR2 PC6400
PSU: 400W

On another note, I have another similar PC capable of running up to 6 screens running an EVGA GeForce 9600GT with an EVGA GeForce 8400GS and a GeForce 6200, same P5QL Pro motherboard but with a C2D E8600 CPU, same RAM, same PSU.

Any help is greatly appreciated!!