Ah - you don't anything powerful to run on a 18-inch monitor. Sorry I don't know Indian shopping sites!
In this range, you should probably be looking for a Radeon HD 5570, 6570 or 6670. GDDR5 on the card is better, but GDDR3 is ok too.
Other cards to look out for: 5670, 5750, 7750 and Nvidia: GTS 450, GT 545, GT 640.
Graphic card hierarchy chart will help: http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html