Thanks a lot everyone for the reply.
I would like to let you guys know that the maximum resolution my monitor limits is 1336 X 768 .
I have short listed Sapphire HD 7850 and MSI Frozr Gtx 560ti , but i am sure that according to my screen resolution even a mere HD6770 or 7770 is enough ( i am aware of that so please dont suggest , i might be changing to a full hd monitor later).
additional info : my mobo i will be getting is ASUS M5A78L-M LX V2 which i dont guess will really support HD 7850 coz its Pci 3.0
so i have to change mobo in that case. Do u think the Gtx 560Ti's or HD 6950 / HD 6970 will fit in this mobo.? http://uk.asus.com/Motherboards/AMD_AM3Plus/M5A78LM_LX_V2/#specifications
Thanks once again everyone