I would recommend holding on until AMD launches their 8000 series (official not OEM) and then wait for the 7870 prices to go down. However if you are buying one in the immediate future then the 650Ti boost is better than AMD offerings at 150. For AMD's best card for around 150 click here and my Nvidia recommendation is here
The performance of hd7850 1gb and 650ti boost 1gb is on par,now u can go to any one of these,but i would recommend u to spend 30$ more and get the gtx650ti boost 2gb version or spending 10$ more and getting the hd78502gb(which would be the winner),now it's upto ur budget friend....