Which is the best graphics card under the budget of Rupees 6500 which my Mercury Motherboard P1945GCM can run?

Your PC supports 5770. Buy Radeon 5770. It is the cheap and best card. Its for Rs 7500. It requires 450 watts smps. Buy this one and it supports all dirctx11 games. This card is also known as evergreen card because it supports all future games. Dont go for nvidia, they are costly than radeon for same performance.