I have 8600 GTS and GTX 570 and tried to run both at the same time connecting to 2 different monitors. I want to buy 3rd monitor to conect 1 monitor and 1 TV to GTX 570 and my old monitor to 8600 GTS. Will it work? I've heard that to run 3 monitors (1 being my tv) you need 2 GTX 570 cards in SLI... Will 1 GTX 570 (running monitor and tv) and 1 8600 GTS (running old monitor) be able to run 3 monitors at the same time or the only option is to have 2 GTX 570 cards?
Yes you can. I'm running three with my main in SLI.
I'm doing 2x 460s which drive a 24" Samsung in SLI and an 8800GT dedicated for PhysX that is driving two 20" Samsungs. From an SLI perspective, so long as the monitor that is for SLI is chosen as your main, you'll be good to go. For your existing setup, as you're not doing SLI, the effect is still the same sans SLI options.
CommanderMuffin is partially correct in the suggestion of how you should connect them though.
I should note that for gaming, obviously, the two other monitors are dead. For driving multiple gaming monitors, they need to be the same size/res...etc.