The 7850 is crappy but then that's all you can afford, which is great if you only play average game in low res/quality.
The reality is that even an OC 7870 (1200 Mhz clock, 1500 Mhz clock, 20% power, 100% fan, 1300 Mhz voltage) still can't play Witcher 2 in Ultra mode. I know it because I just returned it yesterday and got the 7970 Ghz edition.
If you can wait it out, the AMD 12.11 driver is coming out, which may make the 7970 Ghz faster than a GTX 680. And the 7970 is only $200 more. But if you can't wait, 7850 is the best choice, the 7870 will be just as crappy, no need to spend extra cash.