For "proper" Nvidia Surround you must have 2 matching cards in SLI and 3 monitors that all share the same native refresh rate, sync polarity and resolution.
lucky when I purchased the GTX 460 SE Card I got a second with it for SLI (was $110 with rebate, couldn't pass it up and was just getting into gaming). Upgraded to the single GTX 570 SC card later for an HDTV Gamer (570 is amazing, 460 SE's couldn't touch it even in SLI for rendering, but close in framerates), though here lately I got hooked into the idea for triple screen gaming, so to answer your question on the monitors - all three are same spec, LG Flatron L222's and luckily all are VESA mount.
For the meantime I suppose i'll just SLI the 460's, and wait for EVGA to drop the 570's down to around $275 or so and snag one, like March or May timeframe when the 600 series is released I guess? I'd drop bills in a heartbeat for the second card, but I'm ordering the Obutto Ozone Cockpit, Logitech G27 and locating a dash with center console that has about the same geometry as the Obutto, so it may take me till then to get everything ironed out.