it is impossible to get a good graphic card at rs 3500
Anyway this is the one near your price range-gt 620
http://www.flipkart.com/search?q=gt+620&as=off&as-show=off&otracker=start
or go for ATI HD 5570 - couldnt find a link
You canplay older games with this GPU , but for modern games it is not possible (like AC unity etc)
If you want to play modern games i would recommend you to get a new PC , as that rig s not suitable
you CPU is weak too , so using any good graphic card (greater than gtx 750) would mostly cause a bottleneck (you could maybe upgrade to a core 2 quad but still with ddr2 ram and a gen 1 pcie it is hard..)
hope it helps!