If you wait for the next best thing, you will wait forever.
When it comes time to build, pick the best available graphics card that fits your budget.
I doubt that the next gen of amd or nvidia graphics will be out until next year.
If you see that a new gen is likely to be available within a month of your build, then you probably should wait.
In a few weeks, we will see what Skylake looks like and what the Z170 chipset brings.
Until Dec/Jan build time, , spend some time researching a case.