Hi Brad,
You have actually touched on something there that I had been meaning to check out, and you seem like you might know the answer. The card I was thinking of using for additional displays is an Nvidia Quadro NVS 440 (because it's Nvidia, cheap and provides 4 ports from a single slot (I will be very tight for space), it's also passively cooled and since the whole PC will be watercooled and practically silent that is important to me).
It is listed on the NVidia site as using the Quadro driver so would be a different driver than the cards which will be running the projectors (I'll be using 580GTXs). I hadn't realised up till now that there were different drivers for the quadro cards. Does the fact that this uses a separate driver mean it won't work? If that's the case I might have to consider keeping my th2go to put on a geforce type card, as I am not aware of any that are cheap and provide 4 ports, unless you can suggest something.
By the way, if you don't mind me taking the thread off-topic slightly and asking you another question I'd like to ask how I should deal with projector placement. I am considering viewsonic pjd7382 (4:3) projectors which have a throw of 0.61 to 1. Doing the numbers I see that I am just able to place them such that each is exactly the right distance to throw equal size images around a 180 degree screen. The numbers I'm working with generate throws between 39 and 48 inches in a radius somewhere in the 57 to 72 inch range. I see from the spreadsheet that I can't increase the FOV much beyond 180 degrees without the placement starting to become very tight - I had considered going to 225 degrees (FSX can do this by using multiple view windows) of physical screen, then when playing other stuff with a maximum FOV of 180 degrees I could warp the image in to the middle thus sacrificing about 25 percent of my horizontal resolution to maintain the 1 to 1 relationship between game FOV and screen arc and stop things looking stretched. Could I even leave the warping set up as it was for 225 degrees then create a custom resolution to get the game to use 180/225ths of the actual resolution horizontally, or if I did that would the projector just scale that back up and project it across the whole screen? - it looks like 225 degrees actually won't even be possible due to projector placement problems, but I see I might get 210 or something, have you ever tried anything like this?
Although it's a Quadro card, I believe the standard NVIDIA driver installer can see that card too, or the inf can at least be modded to use the standard Geforce driver. Some of this is assumption on my part and from seeing Quadro reference in the driver inf previously.
Personally, I've done nothing over 180 degrees because I setup my system to be as generic as possible for gaming and movies. With how most game camera code is done, anything over 180 degrees leads to either issues or outright crashes of the game. The 225 FOV is something I've worked up the math for previously using 1280x800 projector specs for though. The primary issue is that every projector model is different when it comes to mounting location. Lens throw and aspect ratio are the major dictating factors for that. The only real target for something like 225 FOV would be MSFS as it's the special exception to the camera code issues I mentioned previously.
I guess my questions before attempting to answer are #1, what is your target use for the setup? #2, Will it always be dedicated to that target use? #3, What specific software/game are you going to use?