The GTX 560 Ti overclocks very well.
With a good OC, it matches the GTX 570.
Could you possible provide a site for buying PC hardware in India, if you happen to know one?
The MSI Twin Frozer II that person is selling if great for overclocking,
Since it runs very cool and quiet.
Talk to the person selling it and ask if it works fine,
And confirm if it works.