Monish Chowdhury :
my budget is short around Rs. 4000 inr.....
Then you need to lower what you can expect from the video card unless you buy a used one. If you go used, make sure you see it running in a game before purchasing, many times people sell video cards that are faulty.
The lowest "gaming" video card you'd want to get is the Radeon 6570 or the 7750 for a bit more money. Plus keep in mind your CPU and RAM also affect how well a game will perform. Even a 7750 will not run many of the newer games at mid settings. Games like Diablo III or WOW you can run at medium to high on the lower end cards.