Thanks guys for all the reply. I will actually be going for a new mobo and cpu (amd also) in the near future, say 4-5 months. But right now, I only will change the psu and gpu. And I don't intend to use SLI/Crossfire in the future. Single gpu is good enough for me. And the cards in the price range of GTX 650 and Radeon 7770 are the ones I can afford at present. That's why I am wondering which one to go for.
@Dorado: I couldn't understand what you meant by GTX 650 will be a waste because of my cpu. Can you please explain a bit?