Abhraneel Roy :
Then can i run gtx 780.
I have 450 watt psu.
Can you? -> yes
Should you -> probably not!
Really, it depends on the quality of the PSU, ambient temps, air flow, and how much power the rest of your system uses. Tom's has an article where they ran an R9 295X2 on a 500w psu without any trouble. You might run into trouble getting enough adapters to actually plug the thing in and you could melt PSU wires if you pull too many amps through any one of them.
You could probably make things work, but I wouldn't want to be held responsible when your system catches fire. The are quite a few variables that influence PSU suitability. On the one hand, you don't need to supersize your PSU as so many people (and even manufacturers) suggest, on the other hand a poor PSU can cause serious damage. Your best bet, without learning all of the intricacies, is to use a PSU with good reviews that actually has the connectors necessary for your GPU.