I recently added my second GTX 770. I am running a PC Power and Cooling Mk III 850W with no issues, and I have a lot of accessories and other components attached as well. SLI unfortunately has it little glitches, but in my case it is not related to a power issue.
Second important thing is how much cooling you have around the system where your two GPU's would reside. On a harsh game like Tomb Raider 2013 with all the settings to the max, both of my cards tend to run around 80 to 81C. Definitely noticeable in a quiet home