7970. That's not even anything to think about...
The 680 is a waste of money - the 670 is considerably cheaper, and only the 680 is only 5% faster.
If you can get a GHz ed. for that cheap, then jump on it - CUDA cores aren't better than STREAM cores, just more widely used - AMD's card however, is a monster powerhouse.
That being said, what is this system being used for? If it's for gaming, you'll get equal performance with an i5-3570k, a z77 board (so you can overclock), and 8GB or ram. The setup you have there is only useful if you plan on HEAVY video editing - in which case you might want to go Nvidia. (Look up the programs you like to see if they use CUDA or can use STREAM.)
the system built for 40% Graphic editing , 3D etc and 60% for hard gaming (bf3 , gta v ...)
i still dont sure about the CUDA cores, with system like mine CUDA cores wont be useful or yes ?