Installing Displays #5 & #6 on my workstation

So I get my third dual head card in the mail the other day and after two days of patient waiting I took the plunge to install it.  Well, to make a long story shorter, it took me all day to get working.  Typically you just add the video cards and voila, things work without a hitch, well, not this time. 

 What I thought was going to be an easy thing was definitely not.  Putting the video card in and installing the drivers went fine but when I went to the display properties settings dialog I got a very bogus error message:


The currently selected graphics display driver cannot be used. It was written for a previous version of Windows NT, and is no longer compatible with the current version of Windows NT.

The system has been started using the default VGA driver.

Please contact your hardware manufacture to get an upgraded driver, or select one of the Microsoft provided drivers.


Well, I know this error message is bogus because the video card works by itself in the system and the drivers are WHQL certified.  When I opened up the device manager it stated "this device is working properly." So I know the error was just hiding the true cause of the problem. 

 I started playing all the tricks I know with the video card, swapping the PCI bus order, changing PCI/PCIe slots to initalize 1st, nothing worked.  What worked was any two combinations of the video cards (they are all heterogenous manufacturers & drivers), but never all three at once. 

 I finally came across the holy grail of my problem when reading display driver documentation.  It turns out that display adapters are given a certain slice of memory to load in and if the total size of the drivers exceeds this allotment the driver won't load.  Hence why two cards always worked but the third card always failed.  You can set a registry key to change the default memory footprint and increase it such that you have enough space for the drivers to load.  I first change the value to 16MB, which should be enough for most folks having this problem, and then subsequently to 32MB before I was able to run all six displays on my machine without a hitch.

 So, if you are getting the above error message when going to the settings tab of your machine and have multiple video drivers installed and have already tried Q165318, set the following registry value (please make sure your system's backed up before you play with this registry setting):

 HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\SessionImageSize

 DWORD 0x10