I have 5 x 580 connected to my evga 750 and it runs stable.

Best thing to do, get yourself a Kill a Watt, Home Depot carries them, Amazon as well https://www.amazon.com/P3-P4400-Electricity-Usage-Monitor/dp/B00009MDBU

Measure how much your usage increases when you add a card while mining!

Understand your power supply efficiency and then decide if you have room left to add another.
Good read bout efficiency:
http://www.corsair.com/en-us/blog/2012/august/80-plus-platinum-what-does-it-mean-and-what-is-the-benefit-to-me