nvidia surround, VRI CDUII

I currently use a TH2GO but am thinking about moving over to using nvidia surround using my two GTX580s. I know I will need a third GPU as on the 500 series you can't connect an accessory display to the same cards as the surround displays (you can on the 600 series).

Can anyone tell me if, when running in surround mode on my 3 main monitors for the FSX virtual cockpit, I can undock a window (the CDU) and move it onto the screen on my Vrinsight CDUII? nvidia imply that the "accessory display" is only for non-3d applications, do does an undocked 2d window like the CDU count for this, even though it is being undocked from a 3D application?

If this is possible, then presumably using a cheap card for the accessory display is an option (I'd stay with something using the nvidia geforce drivers, like a 610, to avoid complications from multiple drives). If I did that, would there be any performance impact on the VC view running on the 580s, due to having the CDU undocked onto a display connected to the cheap, low-end card? In any case, I may just buy another 580 so I can tri-sli for other games.

If this will work with a low end 3rd card, I'd appreciate any recommendations for single-slot, passively cooled (or _extremely_ quiet fanned) GPUs, nvidia only please since I'd rather avoid running multiple graphics drivers with the attendant possibility of problems. GPUs with cheap waterblocks available would also do - perhaps an older high-end card that can be brought cheaply second hand complete with a waterblock might fit the bill.