The closest a GPU comes to PS4/XB1 tflops is the Radeon 7790. If you get a 7850 You would already be above the tflops of the next gen consoles.
The 6850 @ 1.5tflops is slightly below the tflops of the next gen consoles which If i remember is 1.84 tflops. Even though the 7850 has less tflop performance than the 7790 the 7850 has 1GB more of memory, more memory bandwidth and double the ROPS.
So the best solution would be the 7850, but by no means would your setup "be left in the dust". The 6850 still has double the ROPS more memory bandwidth than the 7790. It does however have less texture units.