Performance wise gtx660 is better than hd7850.
Even when more AA is applied or even at resolutions like 2560*1600,gtx660 is still generally faster than hd7850 in most of the titles.
HD7850 2GB can be found for around 13.5-15k.GigaByte GV-R785OC-2GD Radeon HD 7850-13.5k
If you can find gtx660 around 14-15k then get that or else get the hd7850 2GB.GIGABYTE GTX 660 2GB DDR5 OC-14.6k
Even when more AA is applied or even at resolutions like 2560*1600,gtx660 is still generally faster than hd7850 in most of the titles.
HD7850 2GB can be found for around 13.5-15k.GigaByte GV-R785OC-2GD Radeon HD 7850-13.5k
If you can find gtx660 around 14-15k then get that or else get the hd7850 2GB.GIGABYTE GTX 660 2GB DDR5 OC-14.6k