how to get 3 monitors on 2 video cards

I have a customer that wants to use 3 monitors. He is currently using a USB device and does not like it, he wants all 3 via video cards. Intel says it works. I have tried several cards on a new motherboard (Intel DG45ID) and Intel says it should work. I have tried both as Nvidia, both as ATI, 1 of each. one PCI-Express and one PCI or boht as PCI, only one card will work and be seen in device manager (based on the BIOS setting).
Vista Business Edition, 4 GB RAM 500 watt power supply Quad Core …