The GPU will draw more power when it is under heavy load. So that might bring you closer to like 450-500w which is why they recommend more wattage.
As long as you dont overclock i dont think there should be any problem. You should be a fair bit over the estimated Wattage, but a 100W is enough i'd say.
There are also some Videos (dont remember who did those) proving that a high end rig (780Ti + 4770K i think) didnt use more than 400W.
My only worry isn't that the PSU is only 500W, it's that it's not very high quality in the first place. On the plus side, it's very unlikely anything will actually break if it gets overloaded. If something were to break it would be unlikely it would be anything other than the PSU, which you'd need to replace anyway. Give it a shot.