As far as I know a build like this would be very hard to do using nVidia cards. nVidias in surround have the bad habit of disabling remaining displays that don't belong to the surround group - hence locking out the 4th screen. I run this thing off a single 6970 2GB modded with a more efficient cooler and it works pretty well. I don't know of anybody running a setup like mine on nVidia GPUs, so I slightly doubt it's possible. The only thing I could think of is making a 4-screen surround group and locking the 3D output of DCS A10C to the upper screens using a viewport in the graphics config.

Your mileage may vary, but from in experience Eyefinity is much more flexible than surround. My rig would possibly benefit greatly from a Crossfire setup though, but I'll wait until the southern islands chip series comes out before thinking about adding another GPU. The support for crossfire in windowed mode is another story though...