If you consider that word “ENOUGH” only for start the GPU then yes its enough but you consider it to Power your GPU is going to Demand when you play games then I am sorry my friend, this Power Supply is a NO GO. Here are simple reasons why which will make it clear to you. First of all GTX 780 requires 600 Watt PSU Minimum but CX500 lacks 100 Watt solid, besides your GPU is OC edition with custom PCB which means this will demand more power than reference design. Yes ofcourse NVidia demands much more power for their GPUs then actually their GPUs demand. But they do it for strong and logical reasons. Power Supply is the most sensitive part of a Computer because rest of the components may not effect on other hardware when they are about to go out but Power Supply does, when it goes out many times it takes some of the hardware along with it. So be careful my friend, CX500 is something which will make you regret one day, this PSU is a cheap one, has low quality components and most of all it has only “80 Plus Bronze Certificate”, Not even Silver, Gold is Far Away… (Forget about Platinum)
Get a 80 Plus Gold Power Supply which’s Peak Power is 600 Watt and with a 42A 12 Rail. This will provide to your GPU a secure power that it’s going to demand.
And One last thing Why you are thinking of buying GTX 700 series???!!! Don’t you know GTX 900 series out now??? This series got 1GB more V Ram over all, with some brand new technologies (Maxwell) and benchmark shows 980 is superior than 780. Get a GTX 980 my friend!!!
GOOD LUCK.