Hello ianoop, do you know how many PCI-e connectors your PSU has? I am going to guess 1 6pin.
Unfortunately, better cards usually need 2 6 pins (or some combination of more than 1 pin).
That being said, a 650ti is a pretty good card (don't expect ultra / max settings). Prices in Canada are that the 650ti is about $150, with $20 MiR, and the 7770 starts at $100 (though the 650ti does come with Assassins Creed 3).
Important to note: The newest drivers for AMD bring performance way up in certain games (12.11 drivers). If you look at Benchmarks, make sure to check which drivers they are using, as it may not be the newest.