Since posting my article about having 4 screens, I've had numerous people contacting me asking how I did it, because when they try to do it they get a "Incompatible display adapter has been disabled" error message. The full message is:

Incompatible display adapter has been disabled
At least one display adapter on the system has been disabled because its driver is not compatible with the driver for the VGA adapter.

..and after a lot of digging here is the reason why and/or solution:

The key issue with this problem is the two different windows driver models. There is WDDM (Used by Windows Vista) and XPDM (Used by Windows Xp). If you have two graphics cards that use different driver models they will not work together, and Windows will disable one of them.

With the two graphics card you use, you can have 2 different variations that will work:

• Two identical cards from the same graphics hardware vendor - for example, two PCIe ATI Radeon x600 cards, each in an x16 PCIe slot.
• Two different cards from the same graphics hardware vendor - for example, one PCIe NVIDIA GeForce 7600 in an x16 slot and another PCIe NVIDIA GeForce 6600 in a second x16 slot.

Notice that the bus type - PCIe, AGP, or PCI - is irrelevant. You could have "n" cards in "n" PCIe slots of the same or different lane widths, or you could have "n-m" cards in PCIe slots and "m" cards in PCI slots. The key point to remember is that all "n" graphics adapters use a single graphics driver (WDDM or XPDM).

What wont work?

• Two different cards from different hardware vendors (E.g. one ATI card and one NVIDIA card)

Do there you have it - you can easily add multiple cards. The cards that I use are:

ATI Sapphire Radeon X1650Pro 512Mb PCIe x16

and

ATI Sapphire Radeon 9250 256Mb PCI

which work well!

Further questions on multi-monitor support in Vista might be answered by this page from Microsoft: http://www.microsoft.com/whdc/device/display/multimonVista.mspx