Which is a best GPU??

What cards are available to you? Does me no good to suggest a 5770 if you can't get it where you live.

Don't know anything about hardware prices in India, but I expect they're generally higher than in USA. 5000 INR are about 90 Dollars, which, unfortunately, can't get you a decent dedicated graphic's card. Well, at 110$, which would equal 6100 rupee, you could get either a Geforce GT 640 or a Radeon HD 7750, but, again, i believe that these cards in India are going to cost even more. For approximately 65$ (3600 INR) you can buy a Radeon HD 6670 DDR3, which at least fits your budget. You can read the review here:
http://www.guru3d.com/article/radeon-hd-6670-review/1

Second hand 5770 would be the best option, but if you want new cards, at 5000 INR I think the max you can get the 6670. Oh yes, just make sure the motherboard is compatible with PCI-E x16 Graphics card and the power supply is sufficient for the card/build (Especially for the 5770).

the 6670 should last you some time playing on 720p. its supposedly the gpu in the 8th gen consoles and in console ports with medium settings it should do fine. also a refurbished 6750 is around $80 in the US, this card would be perfect for 720p since its not that demanding.