Gamer AshRut :
The best card in your range is EVGA GT 630. But I would suggest you to save up for a new gaming rig because your current specs for gaming are very low. You won't be able to run new games.
http://www.amazon.com/EVGA-GeForce-1024MB-Graphics-01G-P3-2625-KR/dp/B008ODZPHI/ref=sr_1_4?ie=UTF8&qid=1399365192&sr=8-4&keywords=gt+620
minimum PSU for a GT 630 is 300w
I was going to recommend something from the same line but then saw the specs from that PC. It will struggle with most recent titles