One more review of exactly the cards you're considering, but in german -google tranlate
The r7-250x (hd7770) wins overall. In some games the gt740 ever so slightly beats the r7-250x, but in those cases it's practically neck and neck. However, when the r7-250x wins, it absolutely slaps the gt740 around.
That Sapphire 7770 i linked is great value and will serve you very well,...