I used to play with a stock 1090T and a stock 6790. In December, I upgraded to a 660Ti Power Edition. The boost in performance was very nice. Completed Hitman : Absolution, Far Cry 3, DiRT Showdown, Assassins' Creed III on max settings with little to no AA (cant really notice the difference). Although the framerate never reached the ideal of 60 (obviously), the gameplay was very smooth except in AC3 (game not well optimized).
The jump from a 6870 to a 7950 will be even bigger since 7950 performs a lot better than a 660Ti, FPS-wise. If you're gaming on a single 1080p screen, get a non-reference 7950, overclock both the CPU and the GPU and you're good to go. You shouldnt need to wait for the newer gen cards with the performance of a 7950.
As for which 7950 to get, get the Sapphire 7950 OC
. It is the best 7950 with its custom cooler and unlocked voltage. You will able to overclock it to 1.0-1.1 GHz on the core easily, which will give you performance similar to a 7970.
As for which brand to avoid, XFX would be it. Seeing how you are currently using an XFX card, I just thought you should know that the current gen cards of XFX are not very reliable (issues with cooling among other things).
But before you go overclocking everything, you should take care that your PSU is good enough to handle it. and that your CPU has a good aftermarket cooler that can keep it cool even under high temps that come with the overclocking.
Hope this helps!