Just putting this out there as an option, but it appears that a GTX 660 will also hold its own against the 7850/7870. Not as overclockable, though. It really comes down to what deals pop up in the near future. In the past few weeks, the 7850 has gone as low as ~$150AR, the GTX 660 for ~$165AR, and I think the 7870 was in the ~$180AR range.
I'm looking at all three of the above cards before xmas, and will likely pull the trigger on whichever of the 3 hits one of those targets first. From a pure performance basis, the cards appear to rank 7850 < gtx 660 <= 7850 OC <= 7870 < 7870 OC.