jb6684 :
Yeah the 5000 series is closing out, but so is the 6000 series as 7000 series is current..... So, you can get some good deals on the older models.
That said 5750/5770 vs 6750/6770 vs 7750/7770 pick the "highest number for lowest price..."
The 5000 series and most of the 6000 series are based on the same architecture (VLIW5), so in most cases there isn't much of a difference between them. The 5750 and 5770 are simply rebadged into the 6750 and 6770. The 6850 and 6870 are actually a little slower than the 5850 and 5870 (because the 5870 was the top GPU in that series, while the 6870 is
not the top GPU in the 6000 series).
The 7000 series and Rx 200 are also based on the same architecture (GCN in that case), but it's less obvious what is equal to what because of the new naming scheme.