gtx 780 and 500w psu

That PSU isn't even 80+ certified, so I suspect it may not be capable of producing what is on its label. Although 80+ tests at an unrealistically cool 23C ambient (inside a PC is likely to be closer to 40C), at least they run at full power, which will cause PSU-shaped objects to blow chunks. In the absence of a competent technical review (e.g. with load testing and waveform analysis), I would not trust the unit you have to run a GTX780.
 
Hmmm, misread something...looks like it is 80+ Bronze, and I found it on the 80plus.org site (so it's not a fake label).
If a PSU calculator reported 470W, and this is 550W, you may be safe; you'll be at 85% under full load (which you won't be most of the time). I'd keep a close watch on system stability, air temperature exhausting from the PSU, and how loud the PSU fan is.
 
A PSU is not something to buy cheat. You need to get a PSU that has quality components and has good reviews etc. Not the one that is the cheapest for the watts .... get something with a good brand name like: XFX, Corsair, Antec etc
 
Also, you ideally want a single 12v rail with as many amps as possible. If you have two rails, that splits the power that the unit can put out two ways and you have to make sure you don't overload either of the rails.
 
On a properly designed split-rail PSU, the OCP trip-point is set such that either rail is actually capable of providing more than 1/(number of rails) x (total +12V amperage). This is also why the rails are not strictly additive. For example, on a PSU with a maximum of 65A on +12V, the rails might allow for a max of 22A on each, or perhaps 25A, 25A, and 28A, but the total is still limited to 65A (not 78A). This way, power is not "trapped" on under-utilized rails, but you don't end up with the potential safety issue of a near-short creating a heavy load that is yet within the limit of a single rail. This could keep the PSU energized, even though an open diode somewhere is glowing cherry red and smoking.