If you want to build gaming PC then it depends on which games you play. Check the specification which Graphic Card is needed to run your game smoothly with its specification sheet on the game website.
GTX 1070 is used by some of the Crypto Miners and Gamers.
If that's on your list that you can get it one.
In the GPU aka Graphics Cards are from two of the most famous companies,
one of which is Nvidia and AMD.
So, people always compare which gives better performance.
Nvidia vs AMD just like comparing Intel Processors vs AMD
What I would suggest you is go to the Computer or Electronic market near your place in your city or town and see which products are available and what's the estimate and see what works well for you.
I love AMD Graphics card also called as "GPU" it's my perspective, see as a Gamer enthusiast which Graphic card or GPU would you like to use.
RE: Help with building a gaming PC