The HX520 is more than enough for your system with one 6950 in it, and it might even be enough if you added a 2nd 6950. Although, I think that you'd have to leave the 6950s at stock and only overclock the 3770K to about 4.5 GHz at the most (this is just a guess based on my findings below).
Here's the power draw of the 6950s in Crossfire under full load:
They found that their two 6950s under full load pulled 329W on their own (not counting the system). The HX520 has a +12V capacity of 480W, so this would leave 151W for the rest of the system when the 6950s under full load. According to this, the 3770K could pull about 156W by itself under full load if it's overclocked to about 4.9 GHz using 1.392V. I mean, their system idled at 84W and then pulled 240W when under load, so the difference is 156W. So to me, this means that if you only overclocked the 3770K to about 4.5 GHz using like maybe 1.35 - 1.36V (this is only a guess), then the HX520's +12V capacity of 480W should be enough if you were to have the 3770K and both 6950s under full load simultaneously.
The eXtreme Outer Vision PSU Calculator has the wrong stock voltage for the 3770K. In order to get it to read exactly 156W with a 4.9 GHz 1.39V overclock, I had to change the stock voltage to 1.155V (from 1.25V). With the stock voltage at 1.155V, an overclock of 4.5 GHz with 1.35V results in 135W which would be a 464W draw on the +12V with the 3770K and both 6950s under full load simultaneously. Raising the voltage to 1.37V changed their calculation to 139W, and raising it to 1.38V resulted in 141W. 141W + 329W is 470W. Again though, this is assuming a simultaneous full load between the 3770K and both of the 6950s.
Now, even if the HX520 ends up being insufficient after adding a 2nd 6950, then the PSU would safely and harmlessly turn off. So if that were to be the case, then you'd just temporarily remove the 6950 and begin shopping for a new PSU. ;)
So, I say that when you decide to add a 2nd 6950, try it out with the HX520. It seems to me like it should have enough power if you leave the 6950s at stock and keep the 3770K to perhaps 4.5 GHz. That way if you have a simultaneous full load between the 3770K and both of the 6950s, then the HX520 should still have enough power.