Hehe, i knew this post was gonna be interesting, welcome Brad :)
I'm not sure this will work in SLI mode like the TH2G will, because SLI mode disables the secondary monitors. As you said, its likely that nvidia could easily integrate this into their drivers, and probably make SLI work, but I doubt it works now.
Ideal config for this would be a strong primary card (8800 GTX) and a cheapo secondary card thats just there to feed the screen output. You could get an old PCIe card used from another gamer, or just buy the cheapest PCIe card you can find in the stores. Spend the extra money on nice monitors.
I noticed another way cool feature in the readme:
You can use the borderSize cfg setting to add a "dead
zone" between the monitors where the monitor bezel is.
While SoftTH attempts to make the game not render to the
dead zone area, it still increases the used render
target resolution. Many cards have a limit of max 4096x4096
textures, so maximum border size in, for example 3840x1024
triplehead resolution is 128 pixels (3840 + 2*128 = 4096)
Currently softTH doesn't support mismatching monitor resolutions, but it doesnt seem like a very far-fetched thing to request. Obviously the author has matching CRTs, so that feature was never important to him. However, you do have the ability to specify whatever resolution you want, so widescreen users should be able to get their native resolution, no matter what it is.