Too much difference between systems to say for sure.
I mean realhardtechx seems pretty ok(safe without getting stupid overkill), but my 650ti has had not issues with a 300 watt power supply neither did the 5770 before it. I think the numbers have also changed a bit over time.
With a lighter system(lower TDP cpu and board without all the extra features), you can get by with less for sure.
I see a GTX 670 recommends 500. I do not think I have every seen much over 350 if I trust the UPS i have, but it has been a while since I tested.
Having some extra power never hurts, it should be more easy on the power supply and allow upgrade room.