Just as a test, I hooked up 3 very different LCD monitors to my GTX 680 to see how finicky 2D surround was. I had a very old Samsung 1280x1024 monitor, a few years old 1200p monitor and my 120hz 1080p monitor. Surround worked without a hitch, other than all monitors using 1280x1024 for their resolution, but that was expected.
According to Nvidia's site, the only requirement is 3 digital connections. I used 2 DVI connections and an HDMI connection.
I have them plugged in via a DVI, DVI to HDMI and a HDMI.
They all support 1080p though..