A single gtx 770 is good for most 1080p games. A 1080p monitor has a resolution of 1920x1080, which is 2,073,600 pixels. So, you'll be good up to that amount of pixels.
Two 1600x900 monitors have 2,880,000 pixels. That is 806,400 more pixels, or 0.39% more. If you get another monitor at the same resolution, you should get another 770. They scale in SLI to give about 75% more performance, so you'd be rock solid with another one.
By coincidence, I'm actually selling a brand new one. It's the Windforce version from Gigabyte. They are $380 new, but I plan to list it on eBay for $350. Let me know by private message if you're interested, and I'll give you first crack at it before I list :-)