5670 and 6670 are not same cards , you can see the difference between them in this chart
oops. . my bad. you are right. its the 6770 and 6750 which are rebadges and identical to the 5770 and 5750.
for him the best card would be either 5670 or 6670 , 5670 cost around 4500 and 6670 around 5500 , for current prices of hardware in delhi check
+1
based on current configuration you will need a complete overhaul. Just a new PSU for a more powerful card might not be an ideal solution. If you can upgrade to a sufficiently powerful GPU then you might run into a CPU bottleneck.
Best bet seems to be to get a 6670, and you should manage 720p gaming (1366x768) at medium settings quite well for a lot of games.