Depends on the game, you should look around for benchmarks for whatever games you are trying to play and compare. The 960 and 770 are about the same in power for some games but the 770 will beat the 960 in others and vice versa; however the 960 uses much less electrical power than the 770.
The next best option would be a new or used 1050ti (newest tenchology and drivers), followed by a used 970 which is the most powerful of what i believe are in your price range. I cant tell you which is best option for you since I don't know what the graphics card prices are like in your country. The 770 may work just fine with your power supply, or it may not. The 770 is a powerful card but its old, it was released back in 2013, where as the 960/970/1050ti are newer but even the 1050ti is about to be replaced.