the 760 can run up to 3 monitors by itself, so you'll be fine with adding a second.
are you saying you want to game on 2 monitors? this is not reccomended b/c any first person shooter will be hard to play, the aiming reticle will be split betwen the bezels.
3 monitor gaming... well depends on the settings. 3 1080p monitors on medium would be fine for 760 SLI. you start getting up to high/ultra, it's giong to chew through the 2gb of VRAM real quick.
just depends on how finicky you are about having everything always on ultra.
760SLI is a great bang for the buck... as two of those = performance of a 780ti typically... so adding a second one is money well spent. however, i'm not sure 620w is going to be enough at that point.
you'd need to google 760SLI power consumption.
also overclocked CPUs and GPUs will use lots more power as well
what you do on the monitors is a tough one to answer.