a single 670. you'll at least don't have to deal with possible issues like drivers for newer games (this goes for Nvidia as well, so i'm not dissin' AMD here), microstutter, etc.
it may not perform as good as 2x7850 but the 670 should be sufficient until you feel like getting another cheaper 670 to run in SLI somewhere down the line. unless you REALLY wanna max out every games in 3 x 1080p monitors