Currently I have a 2 monitor setup.
I would like to add a 32" TV for full screen
view as a 3rd monitor.
My video card is the GTX570.
Do I need to add a 2nd card?

Yes, you do.

Doesn't matter what the card is, though if your card is Nvidia, the other one should be, too, to avoid driver conflicts.

If all you want is full-screen preview, you can look into Decklink or AJA cards, but they aren't cheap. I find a second GPU far more useful, and stable.

ATI supports up to six monitors. ATI Eyefinity Technology (http://www.amd.com/us/products/technologies/ati-eyefinity-technology/Pages/eyefinity.aspx)

Thank you guys for taking the time.