This is the best card you can get for your current budget:http://www.flipkart.com/zotac-nvidia-geforce-gtx-560-ti-oc-1-gb-gddr5-graphics-card/p/itmd4uwpxxgqygfy?pid=GRCD4RC6UMPN9YNG
Best card if you wait:http://www.flipkart.com/sapphire-amd-ati-hd-7950-3-gb-gddr5-graphics-card/p/itmd7th7shkx73ms?pid=GRCD7TGZVWDHDUTZ
Though, if you wait, I'm sure the prices will be different in 2-3 months.
And no, you cannot use your old GPU for crossfire. You can, however, if you buy an Nvidia GPU, dedicate it to PhysX processing.