Buy a second hand card at that budget, maybe you can get a 3850 or 7950 etc. Also ocing on a stock cooler is not a very good idea, especially 20%+ oc's if you want to squeeze the most from the proccy, get a good after market cooler. You will also need to invest in quality RAM's (if I am not mistaken Gskill pi blacks are available at Prime, Mumbai).
That said, you will require stable clean power while overclocking, so that would require a good PSU too. All these things need to be kept in mind before overclocking, lest you spoil your CPU or mobo etc.
But on your primary question, if you want a new card get a 8600gt ddr3 versions ( do not pay extra for 512mb). Or look for some deals in the market section.