you from india where are you going to buy and what are your choices
Radeon HD 4850 GeForce GTS 250 Radeon HD 5750 1GB Radeon HD 5770
these should be the minimum you should consider let us know if you have some choices 4870 4890 are also good cards 5770http://www.tomshardware.com/reviews/gaming-radeon-hd-geforce-gtx,2676-3.html
from lamington road, mumbai.
may choose between Radeon HD 5770 or Radeon HD 4850 .
i will be buying it from shop which may save me $10-20.but availability of card will make difference most likely GeForce GTS 250 will be available as those card are common here.