How to use two PCI video cards ?

I've reached the maximum amount of monitors which I can run off my video card. So I want to run a second card off my system to have some more monitors.

I have a Precision T1600 and I am trying to use multiple video cards with it. One of the cards is in slot 1 (PCIe x16) ; and the other is in slot 4 (PCI x4). If I have both of them plugged in at the same time none of my monitors will come up. If I only plug in one at a time it will run fine. 

What do I need to do to power multiple video cards at the same time ? Is there something I need to disable ? I looked into the BIOS and in the Misc Devices there is an 'Enable PCI Slot" checkbox that I've checked and unchecked and didn't notice a difference. 

I can't find any other settings that might be relevant.